As someone who’s been digging into building a database, this is a bunch of helpful resources! the UChicago chidb one in particular seems particularly interesting if you’re interested in how relational databases work.
The book “Database Internals” is a fantastic resource for getting started. Each chapter has a ton of references for further reading material. One small gripe was it didn’t come with code but it can be supplemented easily using code from the various high quality open source databases.
That’s the second link in the list!