New to the scene, I have some questions

So ive got my ps4, it was on 7.51, I updated it via USB to 7.55 and now i activate it through goldhen 7.55 which is sort of sucessful which is great

1. is there a place where i can find homebrew applications as all my searches haven't been fruitful
2. can i use an external hard drive for extra games (sorry newb question)
3. can i play ps3 games on ps4 somehow, ive seen the tutorials for ps2 games

Thanks!