"build multi-tenancy themselves".... this is a weird thing to say. In the modern day, what product company builds single-tenancy..? Even if a customer explicitly wants it, you can make multi into single easily. The other way around is difficult.
Depends on the app and the context. A lot of apps need multi-tenant access control but not necessarily separate containers etc. In those cases, "adding" multi-tenancy means maintaining an Organizations table, a Roles table, and a table to map them to Users.
Maybe it's just different worlds, but in my world people wouldn't consider an app using isolation via containers etc "multi-tenancy". It would be a single-tenancy application. But I guess there are are hybrids these days with such granular database options in the cloud etc.
