Where can SSISDB actually reside?
If you want to run packaged in the project deployment mode (SSISDB), you need to install SQL Server DB engine on the SSIS box.
If you want to execute packages from files system or other stores, you can skip DB engine.
In both cases you need a license for the SSIS box.

In SSISDB mode you will manage packages using SSMS connection to the DB engine.

Steps how to install SSISDB:
1. Open SQL Server Management Studio.
2. Connect to the SQL Server Database Engine.
3. In Object Explorer, expand the server node, right-click the Integration Services Catalogs node, and then click Create Catalog.
4.Click Enable CLR Integration. The catalog uses CLR stored procedures.

From:
https://docs.microsoft.com/en-us/sql/integration-services/catalog/ssis-catalog?view=sql-server-2017