Effect or transition on trimmed or cropped video

Hello, I can't handle with this problem:
I have background video and main video. Background is under main storyline. I've trimmed or cropped main video, so background video is visible.
Question is: how to add effect or transition on main video only? Every time I add effect or transition it effects also on background.
Please help

It depends on the specific transition. Many effect the whole whole frame and all the layers. Some can be modified in Motiin to not do this, but it depends on the exact transition you’re using.

Or put main videos and transitions into a compound clip.