This is a great game, amazing even, but I have one small issue. When I pick up cartridges, what do I do with them? I know I'm supposed to refill magazines, as other comments say that, but how do I do that? Otherwise, It's a great game, I'm just dumb so I can't understand trhis mechanic. Please, help me.
Are you playing in VR or using Mouse & Keyboard?
Using mouse + Keyboard you first need to holster your gun. After that you can insert/remove cartridges. The default keys should be E and Z.
Using VR you can eject cartridges using the trigger and just move cartridges over the magazine to put them back in.
I hope this helps! :)