How do those who believe that Jesus is a created being understand the verses which say everything that was created was created by him?

An important strain in Biblical Unitarian thought, articulated by Bill Schlegel among others and broadly Socinian, is that the 'all' in the examples in the OP refers to the new creation, i.e., the Kingdom and the new Israel that comes about through the Christ.

To hold to that in John 1:3 requires that the 'beginning' there is the 'new' beginning, related to the Christ's ministry, resurrection, and or ruling of the Kingdom.

For an in-depth argument for the beginning in John 1:3 being the new beginning, see this answer to What are the arguments in favour of the 'beginning' at John 1:1 being the new beginning?, which goes through the prologue's internal evidence, the evidence from the rest of the Gospel of John, the evidence from 1 John 1:1, the evidence from the other Gospels' use of 'beginning', the use of 'in the beginning' in the NT, and more.

Note the discussion there not only of John 1, but Romans and Ephesians. Taking our cue from Ephesians, we can apply that to Colossians 1, which is quoted in the OP.

The OP says

"So if the Bible says that everything was created through him, how is it possible that he is a created being? Did he create himself?"

Note the problem here is not just for those who hold Jesus is a created being, as Trinitarians will hold that not 'all' things were created through him, as He himself or the Father were not created through him. Everyone agrees context delimits the 'all' claims, as context almost always does in speech regarding 'all'.

Conclusion

For those who hold Jesus was a created being, a straightforward response here is that the context delimits an 'all' statement, and in the examples in the OP we're talking about the new creation, and all things in the new creation (that relates to the new Israel or Kingdom) are through the Christ.

From a Biblical Unitarian perspective - those that hold scripture as being wholly sufficient to reveal all truth about God, His son Jesus the Christ and His salvation plan for man made possible through him.

How do those who believe that Jesus is a created being understand the verses which say everything that was created was created by him? We'll examine the verses to see if they mean what OP suggests they do.

Everything became through the word, and without the word did not become one that has become. John 1:3

This verse was quoted as a proof that Jesus is not created. Does it say Jesus? No, it doesn't - it speaks of the 'word' or 'logos' Gr.

When did Jesus enter the biblical narrative? When the logos became flesh.

When was that? as John records in v14 (~4BC) Jesus is simply not mentioned anywhere until this point of his conception in Mary and subsequent birth. Equally, there is no 'Holy Son' mentioned either except in the form of a man who would fulfil various prophecies.

There is no need to add imaginative and special reading into the text. If it says 'logos' it is not saying Jesus. Jesus IS the logos, but only after it became flesh. We can see from 1John 1, John expresses this logos as a 'which' - not as a who or person until manifested in Jesus at his conception.

From this careful reading of the text, we see that Jesus was not 'in the beginning', but the logos was. And with God, which also means not God as God is God.

Colossians 1:15-16 (NIV): The Son is the image of the invisible God, the firstborn over all creation. For in him all things were created: things in heaven and on earth, visible and invisible, whether thrones or powers or rulers or authorities; all things have been created through him and for him.

The context is again, vitally important. Is Paul speaking of a Genesis creation as we read in Genesis 1? No. The context is the new creation, the church, the fact that all God's work in making man would be realised fully in, and only in, Christ. Without Christ - who is the image of God, we cannot be the image of God either. All things are fully realised in this new age in Christ. (Rom 8:29) Same story for Romans 11:36 as OP quoted.

A traditional approach proposes Jesus was somehow born from all eternity. By reading the text carefully we remove the need to make careless assumptions. Note Col 1 carefully, "things in heaven and on earth". God created the earth and the heavens - not just the things ON the earth or IN the heavens. Again, this is not speaking of a Genesis creation so we cannot carelessly insert Jesus into it without creating massive contradictions within scripture.

The OP states, "I mean, that's pretty obvious." (That all was created through Jesus). We might think this but only if we read in that which is not there and fill any gaps we perceive from a faulty premise. Jesus didn't create anything. He is the servant of the Creator, not the Creator. Just as the logos did not create either, but God created 'through' His logos.

He received the spirit from God - making him again - not God.

Here is My servant, whom I have chosen, My beloved, in whom My soul delights. I will put My spirit on him, and he will proclaim justice to the nations. Matt 12:18 (Luke 3:22)

In attempting reconcile these obvious truths about Jesus not being God, a "two-natured" Jesus was devised. That way he can be a man when he needs to be - like be tempted and die, to be flesh and not spirit, to be made heir and exalted to the heavens. And be a God when he needs to be to fit other dogma. This concept is not biblical but a distraction from the biblical truth of the human Jesus, born of Mary with no pre-existence, made like us in every way Heb 2:17 If he is also God - he is not like us at all!

Extracts from; https://www.biblicalunitarian.com/ https://www.biblicalunitarian.com/verses

Warning: These are not my own beliefs, but I also don’t believe in straw-manning so this is only my best attempt to describe in their strongest form, what I might believe if were an Arian.

I think the argument could be made that Arianism and semi-Arianism try to grapple with just these texts. Taking their cue from Colossians 1:15, Arians and semi-Arians both deny Christ is h*m*ousios with the Father - they reject the idea that he is the exact same, un-created substance and instead argue he is the uniquely first among created things. In the Profession of Faith of Arius Arians confess this about Jesus:

He who has begotten the only-begotten Son before aeonian times (χρόνων αἰωνίων), through whom also he made the aeons and everything, who produced him not in appearance but in truth, giving him existence (ὑποστήσαντα) by his own will, unchangeable and unalterable (ἄτρεπτόν τε καὶ ἀναλλοίωτον), a perfect creature (κτίσμα) of God, but not like one of the creatures, a product (γέννημα), but not like one of the things produced (γεγεννημένων)...

Four relevant observations can be made from this: (1) Jesus is begotten before the creation of everything else, (2) in fact before time itself, (3) it is through him that God made time and everything, and (4) although a creature Jesus' creation is not like one of the other creatures, and although a product he is not like any other thing produced.

According to Arius then, Jesus is a sort of 'third thing' unlike the un-created Father, but unlike anything else created in time including time itself. Here is how they might interpret the verses you cite:

John 1:3 Your translation uses the word "become" which makes this verse a slam dunk for an Arian. Becoming involves change or creation in time. While Jesus was created, he was done so in a uniquely nontemporal way. Therefore, Jesus is not something which became, but existing prior to what has become, he can - without difficulty - be involved in the creation of all of it. Even with a different English translation, the greek word ginomai can by an Arian always be translated to involve the creation of things in time.

Romans 11:36 This verse doesn't, on first observation seem to refer to Jesus specifically, the him being a reference to verse 33's use of theos. So I don't think an Arian would be easily forced to thing of this as a reference to Jesus. Although perhaps it is a cross-reference to point out Jesus is referred to in other passages in the exact same way God is being referred to here. Still, an Arian can say this is about God and that similar passages can also be about Jesus since according the the Arian profession of faith:

...the Father gave him the inheritance of everything...

Colossians 1:15-16 It has already been pointed out that this passages seems to support the unique creation of the Son. It is then a matter of reading from the text to feel inclined to interpret verse 16 as including the (uniquely created) Son in the creation of all other things. The exception seems to be demanded by verse 15.

An even stronger explanation for these passages can be fielded by the semi-Arians such as are represented by the 359 Creed of Nike - many believe Sir Isaac Newton was a semi-Arian. They were h*m*iousians meaning they rejected both the Arian claim that Jesus was unlike the essence of the Father and the Orthodox (h*m*ousios) view that Jesus and the Father are of the exact same essence. The Creed of Nike can affirm belief

...in one only-begotten Son of God who before all ages and before all beginning and before all conceivable time and before all comprehensible substance (οὐσίας) was begotten impassibly from God through whom the ages were set up and all things came into existence, begotten as only-begotten, sole from the sole Father, like to the Father who begot him, according to the Scriptures, whose generation nobody understands except the Father who begot him.

Semi-Arians believe Jesus was of 'like' not the 'same' substance as the Father. Here Christ is described as being created impassibly. Impassibility is a term used only of God an comes from negative theology which tends to emphasize God's inability to change or be change, be created or 'caused' in any sense to become. This tells us something about the way Jesus was made uniquely different from the creation of anything else, and at the same time tells us "nobody understands except the Father". This clarifying that Jesus' creation was 'creation' in a sense we are not capable of understanding means that it is only natural to include exceptions for Jesus when dealing with the creation of things we refer to as being 'created'.

In the end, Jesus' being created before time, in a way different from all the rest of created things, allows an Arian to involve him in the creation of "all things" because "all things" shouldn't include Jesus.