Is this like the mags they use in air soft?
Yes, there are many magfed guns with the majority being old technology mechanical guns but with a magazine feed system instead of a top feed hopper, but with other options such as the dye dam which takes electronic technology with a magazine feed but also the ability to mix and match magazine or hopper - or even flick a switch in game to swap as you wish
(That option being aimed at normal paintballs in the hopper and 'first strike' shaped paintballs in the magazine, but the magazines can take either style)
Eye sight test to find my dam:
The hammer 7:
Even mag fed pistols like the Tpx
It's not compulsory to have the air under mounted! More normal to use 12 gram
Co2 or a remote line to air
And not to forget the 'limited paint' options allowed at some magfed games