Where did all the worlds population come from?

According to the Old Testament, man was created by YHWH.

Genesis 1:26 KJV And God said, "Let us make man in our image, after our likeness: and let them have dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth, and over every creeping thing that creepeth upon the earth."

YHWH created Adam, the first man, from the dust.

Genesis 2:27 KJV And the Lord God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living soul.

YHWH made the man sleep, and formed the first woman from his body in Paradise.

Genesis 2:21-24 KJV But for Adam no suitable helper was found. So the Lord God caused the man to fall into a deep sleep; and while he was sleeping, he took one of the man’s ribs and then closed up the place with flesh. Then the Lord God made a woman from the rib he had taken out of the man, and he brought her to the man.

The man said,

“This is now bone of my bones and flesh of my flesh; she shall be called ‘woman,’ for she was taken out of man.”

That is why a man leaves his father and mother and is united to his wife, and they become one flesh.

This last sentence can be taken as didactic. Adam names the woman "Life".

Genesis 3:20 KJV And Adam called his wife’s name Eve, because she was the mother of all living.

Assuming that the narrative is complete, all mankind descends from Eve maternally, though at least in some occasions paternal ancestry might be attributed to other creations of YHWH, for example:

Genesis 6:4 KJV There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown.

Here, "sons of God" is traditionally understood to mean Angels. In general, mankind is considered to consist of the descendants of Adam and Eve. This doctrine that all people are their descendants is crucial to the doctrine of original sin. Regarding the literal reading of the Genesis narrative, consider the following statement from Catholic Answers :

Concerning human evolution, the Church has a more definite teaching [than that of other organisms]. It allows for the possibility that man’s body developed from previous biological forms, under God’s guidance, but it insists on the special creation of his soul. Pope Pius XII declared that "the teaching authority of the Church does not forbid that, in conformity with the present state of human sciences and sacred theology, research and discussions . . . take place with regard to the doctrine of evolution, in as far as it inquires into the origin of the human body as coming from pre-existent and living matter—[but] the Catholic faith obliges us to hold that souls are immediately created by God" (Pius XII, Humani Generis 36). So whether the human body was specially created or developed, we are required to hold as a matter of Catholic faith that the human soul is specially created; it did not evolve, and it is not inherited from our parents, as our bodies are.

While the Church permits belief in either special creation or developmental creation on certain questions, it in no circumstances permits belief in atheistic evolution.

While the Cathlic Answers here diminishes the Church's teaching that all people are descended from Adam and Eve, the doctrine is clearly included in he Catechism of the Catholic Church:

337 God himself created the visible world in all its richness, diversity and order. Scripture presents the work of the Creator symbolically as a succession of six days of divine "work", concluded by the "rest" of the seventh day. On the subject of creation, the sacred text teaches the truths revealed by God for our salvation, permitting us to "recognize the inner nature, the value and the ordering of the whole of creation to the praise of God."

338 Nothing exists that does not owe its existence to God the Creator. the world began when God's word drew it out of nothingness; all existent beings, all of nature, and all human history are rooted in this primordial event, the very genesis by which the world was constituted and time begun.

375 The Church, interpreting the symbolism of biblical language in an authentic way, in the light of the New Testament and Tradition, teaches that our first parents, Adam and Eve, were constituted in an original "state of holiness and justice". This grace of original holiness was "to share in. . .divine life".

404 How did the sin of Adam become the sin of all his descendants? The whole human race is in Adam "as one body of one man". By this "unity of the human race" all men are implicated in Adam's sin, as all are implicated in Christ's justice. Still, the transmission of original sin is a mystery that we cannot fully understand. But we do know by Revelation that Adam had received original holiness and justice not for himself alone, but for all human nature. By yielding to the tempter, Adam and Eve committed a personal sin, but this sin affected the human nature that they would then transmit in a fallen state. It is a sin which will be transmitted by propagation to all mankind, that is, by the transmission of a human nature deprived of original holiness and justice. And that is why original sin is called "sin" only in an analogical sense: it is a sin "contracted" and not "committed" - a state and not an act.

417 Adam and Eve transmitted to their descendants human nature wounded by their own first sin and hence deprived of original holiness and justice; this deprivation is called "original sin".

For some time, all of mankind spoke the same language, until YHWH made them speak a diversity of languages because of the sin they were committing.

Genesis 11:1 KJV And the whole earth was of one language, and of one speech.

Genesis 11:5-9 5 And the Lord came down to see the city and the tower, which the children of men builded. And the Lord said, Behold, the people is one, and they have all one language; and this they begin to do: and now nothing will be restrained from them, which they have imagined to do. Go to, let us go down, and there confound their language, that they may not understand one another's speech. So the Lord scattered them abroad from thence upon the face of all the earth: and they left off to build the city. Therefore is the name of it called Babel; because the Lord did there confound the language of all the earth: and from thence did the Lord scatter them abroad upon the face of all the earth.

From the Catechism,

57 This state of division into many nations is at once cosmic, social and religious. It is intended to limit the pride of fallen humanity united only in its perverse ambition to forge its own unity as at Babel. But, because of sin, both polytheism and the idolatry of the nation and of its rulers constantly threaten this provisional economy with the perversion of paganism.