Most of the fighting in Donbass is in or around the Ukrainian defenses and trenches they've built since 2014. Likewise Ukraine failed their offensive last year because the Russians built their own defenses.
This is the biggest reason the war seems to be slow, and the fact there is way too much urban fighting. Almost every little village needs to be completely destroyed. The open fields both sides capture are easy exactly because because they're a trap for massive artillery and drone attacks.
