The only people who can see devices in your tenant are the primary login that was first used to register with Admin By Request and the users listed in the portal under Logins > User Logins.

The installer file downloaded from the portal is unique to your tenant. Depending on the target operating system, it can be an executable file, a package or a script and it is signed with a license that applies only  to installers downloaded from the tenant in which you are currently logged-in. The same license file is applied to each of the operating system client installers: Windows, macOS, Linux and Server.

This is true for free plans as well as paid plans.

When installed on an endpoint, once the endpoint connects successfully, you will see in real time the status of the endpoint in your Inventory, which is also unique to your tenant. You will not see any endpoints installed with files downloaded from other tenants - this is simply not possible.