I'm confused about /usr/local

I'm trying to help a co-worker in California ... he can't write to /usr/local even if he logs in as super-user. I started digging and I realize that in general I can't write to the /usr directory - because it resides in the read-only root filesystem. However, /usr/local in my machine (Macbook M1 Pro 14 inch running Monterey 12.6) seems to be magically in a different filesystem /System/Volumes/Data/ ... I don't understand how is it that /usr/local is directory that lies on a different filesystem but is not a symbolic link. I have the impression that this is one of the new properties of APFS as the root and the /System/Volumes/Data filesystems are actually in the same APFS partition. If I list it with details (ls -O option) it shows that /usr/local is a sunlnk and documentation just says that sunlnk cannot be deleted or modified unless you're in SIP mode. Can someone help me understand this puzzle? How can my friend write to his /usr/local perhaps creating a new /usr/local? Thanks!