It used to help me as a beginner to vary basic examples and learn through that. Of course, with the help of the available documentation.
Since there are many useful small examples published here in the forum I have compiled them in the Collection of examples from discourse.threejs.org .
Note that three.js is constantly changing. You should start with the 2021 vintage. See the beginner example in the collection and read the docs that can be found at the bottom of the page * discourse.threejs.hofk.de .
The official examples are often very complex and for the beginner it is hard to pick out a single thing.
Good luck 