The latest Jepsen report on particledb triggered a new found interest in the SSI implementation in postgres.

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.

Incidentally, the 2020 Jepsen report on Postgres found a long-standing bug in the SSI implementation [0]

[0] https://news.ycombinator.com/item?id=41560556