Merging APFS Containers using the Disk Utility Application
If you wish to do this in the GUI, you can do advanced hard disk editing with Paragon Hard Disk Manager.
Not "merge" merging the containers, but most likely you'll need to remove a containter and then resize main container where the macOS resides. It can be done on the fly - i.e. you can edit the same partition/container from which you booted the system. Paragon Hard Disk Manager has 10 day trial period, during which you can use all its functions.