macOS Catalina Boot Volume Layout – The Eclectic Light Company

Firmlinks are similar in some respects to traditional symbolic links, but are only made between folders, work in both directions, and with a little added magic effectively merge the contents of two linked folders. For example, the /Users directory on the read-only system volume contains some directories in /Shared. As /Users is a firmlink, the directories there are merged with those in the /Users/Shared directory on the writable (Data) volume.

Source: macOS Catalina Boot Volume Layout – The Eclectic Light Company