How to resize main partition to full available physical space?
One way to do this is to completely reformat the drive. This will collect all sub-volumes together back to the original. This would, obviously, erase all data on the drive, so back up your macOS with Time Machine (so you can migrate the data back in easily).


Start by navigating to Disk Utility in macOS Recovery:

Change the view from "Show Only Volumes" to "Show All Devices":

Finally, select the drive at the root of the tree shown, then reformat it using the "Erase" button in disk utility:




Once you have a fresh drive, you can reinstall macOS using your backup drive and then continue from there.

Hope this help! Please let me know if you have any more questions!