Divine Frequency has two combat systems running in parallel, and understanding how they hand off to each other is the difference between an efficient run and a grind.
The guns follow the classic retro-FPS model - a slot per category, swapping freely, with modifications layered on top. The modification tree is where build identity forms. Because the tree branches on your archetype, a weapon that is strong on one path can be a dead end on another, and the game's balancing leans on the assumption that you commit rather than mix freely.
Ammo is a constraint, but a softer one than in most shooters - the game's difficulty is more about enemies than about a punishingly thin magazine. Where you feel the pressure is in the reality layers, where the enemy density rises faster than your firepower curve.
Psionics are the second half of the combat loop and they are not an alternate fire button. They draw on a separate resource that regenerates on its own schedule, which means you cannot simply use them whenever ammunition is low - you are managing two economies with different refill rates.
Their function is specific. Certain enemy types are resistant to conventional damage and effectively require psionic attacks to deal with, and certain situations - specific enemies in specific layers - are designed to be solved this way. Firing more bullets at the wrong target is the most common way new players lose ammunition they needed.
The strongest play is usually to use firearms for what they are good at - clearing grouped, physical enemies efficiently - and reserve psionics for the targets that require them. The pressure comes from the fact that those moments often arrive in the middle of a fight rather than in isolation, which is exactly when your psionic resource is most likely to be on cooldown.
Learning which enemies need which answer, and repositioning so the fight happens on your terms rather than theirs, is the core skill of the game.