Everyone always says "get down the fundamentals, have solid fundamentals, don't forget the fundamentals, always work on the fundamentals". I understand what the term means, but what exactly is the goal? Am I just practicing the basics? Is there a specific set of skills I should have down? How do I tell what's fundamental, and what's extra?

I would suggest looking at what happens most often in a game and what could happen most often in a game for you.

Catch and shoot oustide shots as as example. That is something that happens often. So that's something you want to get good at.

Passing, dribbling, pivoting... that all happens very often. You want to be good at those fundamental skills.

Close outs on defense, defending the ball.... that happens often.

Regarding extra, coaches just want you to be effective. They want you to be able to handle the ball without turning it over under pressure. They want you to be able to hit shots when you are open. They want you to be able to pass the ball to open teeamates. They want you to be able to defender and move your feet (containing your opponent). They want you to be able to beat your defender off the dribble. If it takes you 20 dribble moves and step backs to do that... that is EXTRA. Coaches don't like that. Instead, have a really believe shot fake, and blow by your defender. Simple moves to get by your defender are actually better than complex moves because it takes less time. Coaches don't like the ball "sticking" every time you catch. They like quick decisions... shoot, drive, or pass... make that decision in less than a second. If you drive, beat them right away.

Here are more thoughts on skills and fundaments:

https://www.breakthroughbasketball.com/fundamentals/basketballfundamentals.html