First of all, thank you for trying the game!
When you defeat an Elite (an extra boss), you’re rewarded with an artifact: a melee weapon that works like the ones in Vampire Survivors: it attacks automatically, so you don’t have to aim or shoot manually. During the run, you find more copies and higher levels of your starting weapon, while Elites drop additional weapons of completely different type.
I hope that makes sense, and thanks again for playing and for the feedback! :D