What is the true nature of karma?

physical thing and not a moral one

Karma is created as part of the Dependent Origination Process due to craving and clinging. In the Since it is part of this process it is neither physical nor moral. In this context becoming is the karmic component. in the context of the 5 Aggregates the Karmic component is included in Formations which is not physical but mental. But morality is positive karma (gives results which is felt as being pleasant when experienced) an immorality is negative karma.

universe is a massive interweaving assemblage

The interactions are complex which only a Buddha shall know for sure. Having said this the universe is a complex interaction of many factors.

immoral actions and exacts a eventual penalty just for that reason

Immoral action does have penalties but there is no law giver. The next moment is conditioned from this moment with the input of fabrications created through volitional actions. Each moment has so much state built in, it is like a granary with abundance of seeds which can germinate at any moment to give results. Much of the this state is transferred from moment to moment. By staving each moment of input you can stop the accumulation of karmic seed.

The word karma literally means action, and action is motion, so it is of physical nature, it is the play of energy. Things (forms) exist as long as they act (inner-motion), and the process is either conscious, or unconscious. Therefore, human birth is of extreme importance.

Karma can be simply explained using the Newton's 3rd law : "Every action has a reaction which is equal in magnitude & opposite in direction." These actions include, thoughts, words & physical actions. Through the way we do these actions, we alter the momentum of the force that we emit in doing them. The return is always even equal to the depth of our intention in doing it. That's why Lord Buddha has asked us to remind the good thing we do in our life over & over again. It will increase the depth of our intention in doing it & increase the momentum of the return we get.

It is true that its not a good bad thing ..... BUT it actually almost sits on definitions of good and bad one on one

What you said is said in almost all places that talk about karma in buddhism and its always in this funny way

they start very bold in stating on how karma is not about good and bad but just the law of karma the way things are .... but than if you continue to read you see that good is almost one by one the same as wholesome and same with bad and unwholesome

*** no one is punishing you (also said almost everywhere that they talk on karma) but you will be "punished" not on purpose when you will receive the effects of your action

it has differences like for instance meditation is considered wholesome karma - and can be considered even better karma than donating money for example

giving money for example is good cause its letting go of attachment - also it developes metta --- which are things which will benefit you

edit : but karma can be unfair (like real life police actually) some stuff you do can be very little and have great effect and huge stuff you do have a tiny effect

what is said for sure is that an unwholesome (akosala) action has bad results and good action good results .... when the time is right (i belive you heard about comparing karma to seeds

the unfair part of karama can be different than good and bad which is if you give 40 you might get 20 or get 4000000 its not tit for tat

You are right in using physical things to explain karma. But reasoning can only get us this far. Reasoning does little to eliminate sufferings. The main wish of the buddha is to have everyone be free from sufferings and be enlightened. Since buddha has seen karma being responsible for sufferings he teaches us not to create karma. And since there many types of karma, there are many different teachings - 84,000 of them. Thus besides besides explaining karma, we must know that we should not create them.

O! Bhikkhus, it is volition - cetana that I call Kamma, -cetanaham bhikkhave kammam vadami

Karma is always mental. But the consequences(Vipaaka) are both mental and physical.