I've heard the same typescript feedback from coworkers. Another commenter already linked to the official docs, I always link them to https://github.com/sindresorhus/type-fest#readme which has links and docs of the built in types and can be installed for more.

I have yet to find a straight answer on how `declare` actually works in the Typescript docs. I had to go to Stack Overflow to learn what it's for.

The TS docs basically just say "use declare to declare [thing]" over and over, and it's only mentioned in the handbook on a page that doesn't seem to be listed in the handbook menu anywhere.[0] You have to search for it and wing it, because the search result for this just says "By Example." Notice that there is no article highlighted in the handbook menu on the left like there would be on any of the pages listed there.

[0]: https://www.typescriptlang.org/docs/handbook/declaration-fil...