There is nothing to be made up in my mind. And there is no divide what fundamentals are.

You should know variable, loops, data structures, ... basically core concepts that translate into any language. A language is just syntax once you got those down and you can transition quickly.

If you are relearning those because you switched from JS to python or to c# or w/e then I don't know what you are doing.

And you can learn all of those with any language.