Lol as an English native I don’t realize how confusing it can be. They’re all pretty much the same.

Below par (or sub par) -- not up to standard. a bad thing. Under par -- used in golf. You took less than the average number of strokes for that hole. A good thing. My advice is don't use them interchangeably.

Both are correct

As the above people have said, both are correct but usually we would say (something) is subpar which means below the expected standard(s).

There's also "subpar".