MacBook mirrores my iMac

I’m not entirely certain what you’re reporting is being mirrored; the storage, or a display.

Storage mirroring:

Add your Desktop and Documents files to iCloud Drive - Apple Support

See: Turn off Desktop and Documents

Display options:

Universal Control: Use a single keyboard and mouse between Mac and iPad - Apple Support

Use an iPad as a second display for a Mac - Apple Support

Okay, so this is storage and not display mirroring.

You have enabled what used to be called iCloud desktop.

That was the first guess, listed above. Turn that off.

Follow the linked procedure above to disable (and this with your existing contents), or you can wipe the Mac and start over without enabling that and without restoring a backup to become a completely distinct Mac sharing the same Apple ID.

There is a third option here, as macOS is fully multi-user. You can have a separate login for a different profile, and that login can be managed and operated completely separately, either with the same Apple ID and no iCloud desktop (for instance), or with a different Apple ID and no sharing (for instance). Multiple logins works great, so long as you have the storage for it.