So what does mac do with those ?

Interestingly that created its own annoyances for some since normalization occurs at a different stage.

Unicode and UTF-8 unify many Traditional & Simplified Chinese Characters as well as Japanese Kanji and Korean Hanja.

There were some extremely angry reactions to Han Character Unification[1], though efforts have been made to overcome many of the initial complaints.

[1] "Why Unicode Won’t Work on the Internet: Linguistic, Political, and Technical Limitations" http://www.hastingsresearch.com/net/04-unicode-limitations.s...

I don't believe that's literally true, some all legal and correct Japanese unicode sequences will be transformed under at least some of the unicode normalization forms, no?

Not the case for Arabic. There's hundreds of ligature code points, for example ﰻ (kl), ﷵ (ṣl'm) and ﷳ (akbr). And of course ﷽