- Characters can’t be extended. Classes can be extended in FE8 if you apply EMS, all you have to do is enable it in options.
- The battle animation editor is where you edit those. Otherwise, the question is vague.
- In FE7 you can set them from the character editor. For FE8 there’s a patch that adds that functionality, and you can set the animations in the Editor version of the patch. Otherwise, the question is vague.
- I’m not sure I understand the question, but if you’re looking for patch recommendations there’s this topic. What are the FEBuilder patches considered "QoL"
If you mean editing unit stats in-game, then use the Set Unit Status patches. - It’s been a while since I downloaded anything from there, but I used to use DownGit and never had problems with it.
All that said, I recommend reading this topic as it addresses common questions and gives newcomers advice: UltraxBlade's #febuilder-help FAQ