If you are just skimming this doc, The HoT update path is the section I would jump to.
Understand HoT and then read the zHeap implementation notes (also of TOAST) puts a good mental model of what postgres does and what it doesn't do.
Wow so ahead of their time.
(And since I'm bad at sarcasm and don't wish to mislead, SSI = Serializable Snapshot Isolation in this context.)