You can have multi-tenant applications, and you can have tenants with guest users from other tenants. You just can't have users isolated to one tenant who can access resources in another without those users also being guests in that other tenant.
https://learn.microsoft.com/en-us/azure/active-directory/develop/single-and-multi-tenant-apps
https://learn.microsoft.com/en-us/azure/active-directory/b2b/b2b-quickstart-add-guest-users-portal