I used the 1.16.5-36.1.0 mdk (copy the gradle folder, the buildscript, "gradlew", "gradle.properties" and "gradlew.bat" just like the description in the docs told me, replace "com.example.examplemod" and "examplemod" in the build file).
I imported my folder by right-clicking and selecting "open Folder as IntelliJ Idea Community Edition Project". The project wasn't edited until building and indexing finished.