How does Visual Studio 2019 find my JavaScript/TypeScript modules?

I'm trying to find out what the magic is in Visual Studio that enables it to find my own modules in my project. In my project I have re-mapped the locations using index.js for a Durandal project, and it seems mostly to work, but now I read about a tsconfig.json that seems to have very similar contents to what my app needs. Do I need both? What files is VS probing and reading and making assumptions from when calculating intellisense etc?