You are really looking for architecture diagrams. These are extremely rare in most projects, open source or otherwise.
The reason you don’t see a lot of documentation on algorithms used or architecture is that most of the time the code is not actually novel. It’s like asking a plumber to describe the physical properties of the pipe they used on a job. They’d say “schedule 40” or “copper” and a dimension. They would not describe the manufacturing process or chemical composition of the pipe. The materials are pretty standard and only require special descriptions for when and why they deviate from those standards.