Mongodb has very good documentation -- you dont have to study line by line. Just try to understand and take notes on what it can do, that when you need to use those queries, operators, indexes... etc, you'll know where to begin reading.
Try to build a todo app. But don't follow any tutorials, just try to do it asking questions in stack overflow, medium articles, etc... Just npm init and use postman or some other API testing method of your choice.
In the process of doing the todo app, you'll need to begin constructing models. That is the time when you'll research Mongoose. From there on... its practice, practice, practice.
Hope it helps
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Hope it helps