An added volume on an APFS drive shares space with other added volumes on the same container, and each volume does, in fact, operate/act like separate drives. (And, they share space among the volumes in the same container, so there's probably not a circumstance that you would need to "change sizes on the fly", unless you have gone very wrong in your space needs in the first place
I have a drive here, (which is an SSD) one that I use a lot -- with 8 "normal" bootable partitions (each are Mac OS Extended (journaled), all OS X systems from 10.5 to 10.12, each system is a minimal installed system on 32GB partitions.
That takes 256GB of that 512 GB drive, each separate, and bootable systems.
The other 256GB of the device (which is an SSD) is a single container, with 8 more volumes, all sharing the same space in the container. Each is another bootable system from 10.13 to 12.3. Yes, that's only 5 systems, but after 10.15, macOS systems have more than one partition.
So, 13 bootable systems, all on the one drive.
A bit complex if you look at the partitions from the terminal, but no problem in actual use.
(I use it for boot testing, so I always know I have a system on that drive that will boot any Intel Mac, and I can move that drive to a Firewire enclosure to boot some older PPC Macs (I hate the firmware commands to do that from USB, and I know that I can boot from Firewire)
