A blog article with an overview of modern variants of B-Tree data structures.
https://erthalion.info/2020/11/28/evolution-of-btree-index-am/
A good explanation of trade-offs between hashes and b-trees: https://www.evanjones.ca/ordered-vs-unordered-indexes.html
Written in simple language, well-scoped and short. It's destined to become a piece people link to all the time.