Labels and custom icons on files are stored in extended attributes (implemented as named forks on HFS+; not sure about APFS but the OS presents them all as xatt

Labels and custom icons on files are stored in extended attributes (implemented as named forks on HFS+; not sure about APFS but the OS presents them all as xattrs regardless of FS) on the files themselves in APFS and HFS+. Custom icons for folders are stored in a special "Icon?" hidden file, but not actually in the file's normal data-- they're in an extended attribute too. (The xattr in question is called "com.apple.ResourceFork" and I'm presuming is structurally similar to a resource fork in classic Mac OS.)