https://docs.obsidian.md. You can suggests change or make pull requests here: GitHub - obsidianmd/obsidian-developer-docs.
Personally I think its very underdocumented, like I have to do some looking of how to do this or that with JS when the docs don’t explain it. I actually think its amazing that Obsidian has lots of plugins in spite of it. Perhaps it due to Obsidian only have a few employees. I think more plugin devs should spend the time to contribute to documentation to further give back to the community, to make things easier for other devs.