New to minecraft modding and try to understand what can I do with forge

[MagicWheel]

Hi, everyone! I'm new to minecraft modding stuff and want ask you some questions.

1. I checked forge references and saw there com.mojang and net.minecraft (not net.minecraftforge) and as I understand it's original game packages with all minecraft logic inside including renderers and so on, right?
2. Does it mean that forge has a limited set of instruments which doesn't cover all the aspects of the game? If make my question more specific then does forge provide such instruments that allow me totally change minecraft itself, like base mechanics and etc.? Or I have to use "original game packages" to implement such things?
3. I actively learning basic concepts with forge documentation and tutorials. So in my plans make different inventory system like in diabloids. Is that possible with forge?
4. It is last question related to the second one. So how deeply I can change minecraft with forge?

I guess all my questions above because of that I haven't globally understanding what forge is and how it works inside and how it works with minecraft.
It would be great if you provide some links or topics about it or explain it by yourself but I guess it's to big to be explained in that post at once.

Anyway, thank you all for any help!

[TheCursedModder]

Forge is probably the most feature loaded mod loader currently. You can change just about anything with the game mechanics by using mixins. You can also add more items and feature than what the docs show you. It is good to start with a Modding tutorial series like this one by Kaupenjoe.  Some good examples of what is possible with forge is clockwork, dynamic trees, and I guess create but its on fabric too.