> Question1. What is most common use case for having application root and application container PDBs ?

Sharing the data model of an application among multiple SaaS-customers,

providing separate development enviroments (with the same data model) for different developers,

etc.

Please see the docs for more information:

https://docs.oracle.com/en/database/oracle/oracle-database/18/multi/overview-of-the-multitenant-architecture.html#GUID-4ED7F2E2-C649-47C0-B92A-E0050DED252D

> Question2. Can a regular PDB can be plugged in to an application container in 12.2 ?

Existing applications (in a PDB) can be migrated into an application container;

please see https://docs.oracle.com/en/database/oracle/oracle-database/18/multi/administering-application-containers-with-sql-plus.html#GUID-D2F30723-9E70-469C-8048-CB06DE00173E

> Question3. Can an application root can be unplugged from a CDB and plugged in to another CDB ?

Yes.

Please see https://blog.toadworld.com/2017/06/26/oracle-multi-tenant-application-containers-part-i

for more information on application containers.

HTH

Markus