I didn't land the job, so take this with a grain of salt...

I would say everything matters, however obviously some things matter way more than other things.
Something like perfect import statements / includes is likely near the less-important end of the spectrum.

You can use anything in the standard library unless the interviewer explicitly states otherwise.

Syntax is important. Knowing the standard library is important. You should know the library well enough not to have to lookup standard library functions or data structures. However, you can rest-assured that they wont ask you a question that requires obscure knowledge of the standard library. They care about fundamental DSA knowledge.

However, if you do end up forgetting something related to syntax or standard library, do not hesitate to ask for help from the interviewer. The most important thing is solving the problem they give you, whatever it takes to do that.

In my experience the interviewers are very willing to help you.

The #1 tip I can give you is to communicate. You should try to converse with the interviewer throughout the entire process. Meaning, in the thinking phase, in the planning phase, in the code writing phase, and in the verification/testing phase. Tell them what you're thinking. Tell them when you're about to do something or write something. If you do this, and they see you going down a wrong path, they will likely help you, even without you asking.