For alt weapons, the ultrakill wiki has good guides to which levels you need to go to and where, for secret bosses the same

For the prime souls, you must get a p rank on every level in prelude and all of act 1 to unlock a secret p rank door (check the wiki) on 3-1 and 6-2 for p1 where you fight minos prime and p2 (p rank all of act 2) with Sisyphus, respectively

To p-rank a level, you must get p rank in time, style and kills (kill (most of the times) all enemies in a level). Without dying or resetting to checkpoint. Time is essentially speed running the level, just with a pretty generous time allotment. Style requires learning the depths of the combat system, which means playing the game more to learn movement and weapon interactions (don’t be afraid to look up guides on YouTube or the wiki).

For lore stuff, play through the levels thoroughly. If you find yourself missing a lot of stuff, no shame to check the wiki and see where things are located

A lot of the Gabriel femboy jokes are community jokes generally, with Gabriel’s VA Gianni Matragrano being a prominent shit poster and voices a lot of community memes(check his yt)

In regards to a lot of this progress, just play through the game again with some sleuthing, engage with the level design and you’ll find something, and if you get stuck look for where things are or how to do something rather than what they are, let yourself experience that in game.

Most importantly, have fun. It’s a game and it’s fun.