The heap is described in 1.3 [1] and MVCC is described in Chapter 5 [2]. I'd also recommend the Postgres docs [3], they aren't quite as accessible as this but are detailed and overall well-written.

[1] https://www.interdb.jp/pg/pgsql01.html#_1.3.

[2] https://www.interdb.jp/pg/pgsql05.html

[3] https://www.postgresql.org/docs/14/internals.html