The signal protocols are well documented (https://signal.org/docs/). But their code and architecture? I'm not aware of any docs in that regard. For example, an overview of the code structure is missing. If I wanted to know how the database is encrypted in their apps, do I find an overview on that? Where is the architecture documented? Are the design choices documented anywhere?
You might say, "read the source". I did that a few times for certain parts of the code (in their Android app), and it has very few in-code comments.
Now of course it's perfectly fine for Signal to publish their code this way. People with the specific developer experience will still be able to find the code they're looking for after some searching. But the claim "the code is well documented" seems like quite a stretch to me. I'm pretty sure such docs do exists somewhere, otherwise onboarding new developers would be quite hard. But they're probably not published?
(If there _are_ public code docs, links would be appreciated.)