[vidarh]

While that may be true for some of them, a lot of databases are far simpler.

Sqlite for example is conceptually very simple, and it's fairly cleanly layered, so you can aim to understand the parser, the compiler and the bytecode execution engine separately.

It's also exceptionally well documented. Look at the "Technical and Design Documentation" section here [1].

It won't tell you everything about how a more complex database like e.g. Postgres works, but it will give a very good overview of most of what is relevant for a database user that wants a better understanding of why a database does what it does with a given query.

[1] https://sqlite.org/docs.html