Mac storage basics · System Data and the storage categories
What does "Other Volumes in Container" mean in Storage settings?
Short answer: Space used by other volumes sharing your startup disk's container — mostly macOS's hidden Preboot, Recovery and swap volumes, plus any volume you added yourself.
It's space used by the other volumes that share your startup disk's APFS container — mainly macOS's own hidden volumes (such as Preboot, Recovery and the volume that holds swap), plus any extra volume you added yourself, like a separate volume for a test macOS.
The system ones are managed by macOS and shouldn't be deleted. If you added a volume you no longer need, you can delete it in Disk Utility, after copying off anything on it.