M365 Manager Plus' concept of virtual tenants is segmenting users based on domain, group membership, job location, and other user attributes for administrative requirements. Virtual tenants is what an OU is to the on-premises AD. Administrators can create virtual tenants pertaining to a specific department, team, project, and every possible user group, and delegate management operations on them to technicians.
Virtual tenant delegation is disabled by default, and once enabled, it cannot be reverted. On enabling virtual tenant delegation, domain delegation option will not be available. To delegate domains, admins have to create new virtual tenants with domains added to them, and delegate those virtual tenants instead.
Refer the image below to know where to find the option to enable virtual tenant delegation.
Note: Refer this document for the steps to create a help desk technician.