In macOS Catalina, Apple introduced another new concept to the APFS filesystem: volume groups. This is more of a conceptual grouping of volumes within an APFS container, not a new sub-structure. Apple also greatly expanded the number of roles available for APFS volumes (now there are 16 unique roles). When you upgrade to Catalina, your current macOS system volume is renamed, e.g. to "Macintosh HD - Data", its role is set to Data, and then a new volume is added to your startup disk's APFS container with the System role and simultaneously grouped with the Data volume. The two volumes within that group share special bonds and receive special treatment from the Finder and from each volume's filesystem. From the user perspective, these two volumes are treated as a single, unified volume. If you take a look at Disk Utility, however, you'll see the two volumes as distinct, separate items.
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firmlinks
To pull off the illusion of a single, unified volume, Apple added support to APFS for firmlinks. Like the name implies, a firmlink lies conceptually between a soft link and a hard link. That probably doesn't make them any more clear
