Varies from case to case, usually it’s a multi step process. Initially a bit of logic from the designers/coders (I.e an SMG should have lower long range accuracy and less damage per round than a sniper rifle), some of the balancing refinement is then done via play tests (sometimes in the form of Alpha or Beta test releases). Even then there are typically still things not considered, so final adjustments are made once the game is released by looking at player statistics. Unfortunately it’s a very difficult process without a lot of player data so usually comes down to balance patches after release.