I am trying to expand the system storage by "permanently" adding a thunderbolt storage unit to my mac. I am looking for an solution to create an APFS volume on the external drive and add that to the system APFS container, similar to what you can do in linux (adding two physical volumes to one volume group). Please point me to the right direction as I am lost here...
APFS containers can be composed of one or two partitions. These partition must be specified when the container is created and can not be changed afterwards. You can change the size of the partitions (and thus the container) after the container has been created. Enlarging a partition is subject to the amount of free space available immediately after the partition.
The Linux concept of "adding two physical volumes to one volume group" does not exist in macOS. At least not without destroying the contents.
I have a 2018 Intel Mac mini with a non-removable 256 GB internal SSD. After using the Boot Camp Assistant to install Windows on the internal drive, it became apparent 256 GB of storage was not going to enough. In 2019, a 500 GB Thunderbolt 3 Samsung X5 external SSD was purchased. After looking at specifications, it was determined this drive would operate at slightly lower speeds over the internal SSD. The decision was made to leave macOS Mojave installed on the faster internal SSD and move users home directory (folder) to the new sightly slower external SSD. Below are a few comments about making the move.
The Disk Utility application was used to erase the new SSD. I used the format APFS and named the new APFS volume Puffer.
If you move all users home directories to the external drive and for whatever reason the external drive is unavailable, then you will find it difficult to login. To prevent this from happening, I created a new administrator account name Administrator whose home directory will remain on the internal drive. Once logged into this Administrator account with all other users logged out, all other users home directory can then changed to the new external drive. For example, the image below shows my home directory after changing it to be on the external SSD APFS volume named Puffer. (This image is from macOS Mojave.)
You can copy your files from the old home directory to the new home directory, then remove these files from the old home directory. Or you can choose to keep files in both home directories. You can log in to the Administrator account and change the default home directory to be on either the internal or external drive.
The Samsung SSD was plugged directly into a USB-C port on the Mac. A USB hub was also plugged into the Mac using a different port. Any flash drives or SD card were attached using this hub. This helped prevent accidentally unplugging the Samsung SSD while booted to macOS. However, occasionally the Samsung drive did get unplugged while boot to macOS. I can report unplugging did not result in data corruption. In other words, the Mac was restarted and
was used to verify the APFS container on the external drive was OK.