In a seed round, you presumably don't have your investors on the board, so the answer is ultimately you set your salary. I might have an information right as an investor to certain financial docs which would let me figure out the number for founders I invested in, and I'm positive they'd just tell me if I asked nicely, but I don't recall any formalization in the docs about it. (Sample size: 2.)

Common practice is to pay oneself substantially below market, with higher salaries for entrepreneurs with higher personal expenses, which usually translates to "married plus kids plus house equals a lot more money." I'm unsure common practice is either fair, wise, or optimal, but not strongly committed to that that. (I do think strong pushback on any reasonable founder salary is a disqualifyier for investors that I'd take money from or advise taking it from.)

This answer is modestly more complicated after you have a board, presumably after series A.

So basically investors want to know that you are not taking investment because "doing a startup" pays better than your market-rate job. It's a motivation filter.

Below market salaries motivate founders to generate profit. Since investors know they have legally little control over the founders, any motivation increases investment value.

The board of directors decides.