Apple has specific advice on making additional volumes on a backup volume.
In macOS Big Sur, you can finally use APFS-formatted volumes as Time Machine destinations. They aren’t backwards compatible with macOS Catalina or earlier versions of macOS, and they require erasing an existing HFS+ (Mac OS Extended) disk to reformat—you can’t convert HFS+ to APFS for Time Machine and retain the drive’s data in place as you can when upgrading your macOS startup volume to High Sierra (SSDs), Mojave (Fusion Drives, HDDs), or later.
However, what if you want to share that APFS drive, dividing space between Time Machine and other purposes? Apple has quite specific advice on how to proceed: add a volume, not a container.
A brief refresher: APFS is Apple’s modern, SSD-optimized replacement for the once-modern filesystem that’s been in use for many years. APFS is much more sophisticated than HFS+, allowing more flexibility for how data is structured and kept secure and separate, as well as offering particular features suited to SSD. The units of measure on an APFS formatted drive are containers, which are collections of volumes. In HFS+, drives were partitioned solely into volumes. One way to think about it: HFS+ was a carton of eggs, each egg a volume. APFS is a box that contains cartons of eggs.
Read more at MacWorld.com