Yes, they need to be in stems to adjust the individual levels, there are lot of multichannel audio formats out there now compressed and uncompressed.
If you look at UJAM Virtual Drummer it is the recordings of live real drums being played by a session player same as RealDrums but gives you individual level control as well as GM compatibility to add your own custom fills, rolls, riffs or solos to the real playing of the drummer.
Biab RealBand needs this !

https://www.ujam.com/solid/
https://www.ujam.com/heavy/
https://www.ujam.com/phat/