Daniel Lemire has some very interesting blog articles about "low level" data structures that you'd think you understood in first year CS.

I just linked to a google search on "hash tables" because that's what you asked for, but some of his other posts are really quite fascinating.

https://www.google.com/search?q=site%3Alemire.me+hash+table

I don't know if you'd call him a theorist, but I think this gentleman maintains GHashTable and I found his posts comparing/benchmarking widely used hash tables really informative and entertaining:

https://hpjansson.org/blag/2018/07/24/a-hash-table-re-hash/

I wrote some hash tables at an automated trading firm. They were really only suitable for our exact access pattern though.

Usually you find people like this making libraries or programming language runtimes I guess.