[SapMan]

1) AddCharModel
2) AddSimpleModel

How those two functions are used, and what is the difference between them. If I want to add a weapon or a custom skin that I should use and how?

[M8]

AddCharModel is used to add a custom skin. AddSimpleModel is to add objects. See this tutorial for more info; https://sampforum.blast.hk/showthread.php?tid=644123
Also, you cannot add custom weapons.