Their brightness is a mixture of a lot of things, including the huge PV arrays and the angle they have with the sun when they cross the terminator between night and day.

Starlink have already put a lot of effort into their satellites being much less bright than most satellites, including tilting their PV away from earth during the terminator crossing, so from what I've read you'll mainly see them while they're being deployed and while de-orbiting.

(Part of my still-expanding draft blog post about space data centres is to work out how bright a million much larger objects would look. If they were in the orbit with the most sun, that's a terminator-following sun-synchronous orbit, which is maximum brightness).