if i buy asset from the asset store

it’s environment 3d asset, can i use the demo scene included? (in my game for commercial use)

Yes, as long as the asset creator has used all original and/or properly licensed assets in the scene. Redistributing assets is not ok ie. using the scene for your own asset to be sold on the asset store, but for use in a game it’s fine.

Everything included in the asset you buy is clear for commercial use in a game.

As far as I know it’s the responsibility of Unity and/or the asset creator to make sure the asset you buy is clear for commercial use, so you shouldn’t worry about that.

More info here: Use of asset store assets in commercial game. - Questions & Answers - Unity Discussions