The official line from Putin is that all Russian soldiers fighting alongside the independentists are volunteers, but that sounds as true as his earlier lie about the "green men" being local militias.
This is correct, but there were large independentist brigades comprised of volunteers. Indeed that's how this civil war started.
Now there is a formal army of "unrecognized republics" formed from same volunteers plus a lot of locals who are just in it for the pay. The morale is correspondingly much lower.
AFAIK Russian army did some heavy lifting but not on the regular basis. They merely protect "republics" from collapse.
The official line from Putin is that all Russian soldiers fighting alongside the independentists are volunteers, but that sounds as true as his earlier lie about the "green men" being local militias.