Where in Active Directory does a Linux host appear after realm join, and why there?

In the default Computers container at the root of the domain, because that is where new computer accounts go unless they are created elsewhere first.

realm join creates the computer account without a target location, so AD puts it in the default container for computers, just as it does for a Windows PC that joins the domain.

The consequence is the same as on Windows: Computers is a container, not an organizational unit, so no Group Policy can be linked to it and no administration can be delegated on it. The Linux host should be moved into the right OU of the designed structure, or created there in the first place with realm join --computer-ou="OU=Linux,OU=Computers,…".

Go deeper:

• Microsoft Learn: Redirect the users and computers containers — why new accounts land in the default containers.
• Debian manpage: realm(8) — the --computer-ou option of realm join.