I know about APFS containers and volumes, and actually you misspoke in this particular instance, because a container's size is fixed, same as that of an HFS+ partition; it's only volumes that may expand and contract as needed.
You are correct, a Containers size is fixed by the size of the drive or drive partition, it is an intermediate virtualization layer between the hardware and the volume or volume group.
If we knew what it was we were doing, it wouldn't be called research, would it?
