Would recommend reading Ben Edgington's annotated spec: https://benjaminion.xyz/eth2-annotated-spec/phase0/beacon-ch...

It's not entirely up to date (none of the annotated specs are) but out of the three annotated specs I'm aware of, it's the one that is most focused on explaining how things work. Vitalik's annotation (linked in another comment) places a particular focus on the reason specific constants where chosen.