Only the admins can see all the user accounts on the web UI page “Settings > Users”. So all users cannot see each other from there, but if a user searches for another user which has been provisioned to the instance, they will be able to find said user.
No. What I meant by instance is a complete NC install. For instance, let’s say you have a distributed system comprising of an APP server (NC), redis server and a mysql server, be it VM or physicals, all of these machines serving the group of users from Company A, is seen as a single NC Instance.
So one Nextcloud instance, serving Company A, can comprise of a single, or multitude of servers (tenants).
You can create a VM for Company A that houses a complete Nextcloud install. Then you can create another VM to serve users from Company B. hth a bit.