Id like to start modeling and texturing holsters for all kinds of weapons
but my question is, are the weapons that are shown when holstered on the back or hip or whatever a seperate mdoel
or is it just the weapon itself put on the holster location?

It is the weapon itself. Holsters will have to be their own "armor" item and self equipped. Sheaths don't really work, and sure as hell not for guns.

Considering guns can have backpacks (minigun, grenade machinegun, etc) why can't rifles use the backpack slot as a holster?

It might not look good holstering/unholstering the weapon (would probably clip right through) but might look cool while holstered.