macOS has an elaborate set of rules determining which types of xattr are preserve during different types of copying, including syncing to iCloud Drive. Here they are in full detail.
HFS+
Extended attributes were added to Mac OS X 10.4, and soon supported the quarantine xattr. They have since flourished, and have valuable properties.
Released in Mac OS X 10.5 Leopard on 26 October 2007, it supported Time Capsules launched in January 2008, and in Big Sur could back up to APFS.
Originally two separate apps, they were brought together in Mac OS X, and have survived largely unscathed to Sequoia. Here are some highs and lows to remember.
Sparse files are now common among databases, disk images, and virtual machines. How they work in APFS, how they’re created, and how they can explode.
Classic TM backed up HFS+ to HFS+; current TM backs up APFS to APFS. But what if you want to back up a mixture of APFS and HFS+ volumes?
How the resource forks of Classic Mac OS became extended attributes in Mac OS X 10.4, then flourished. How clone files handle xattrs, and which are used by APFS itself.
From the GPT division of storage space, through APFS containers, down to individual volumes, an account of how APFS works.
Introduced in iOS 10.3 on 27 March 2017, then in macOS 10.13 six months later. It ups and downs, and where it still has further to go.
B+trees, directory records, directory and file names and Unicode normalisation, and whatever happened to the promise of fast directory sizing?
The Eclectic Light Company 