On my R1T When it’s set at highest in all purpose it will lower at 45mph. At least that’s what it does for me. I don’t think theres an auto selection or at least I haven’t had to use it.
The Rivian will only lower itself from Standard to Low when it's set to auto ride height. If Auto ride height is on, it will switch to low once you reach about 55-60mph. When you slow back down, it will switch back to standard ride height for best ride quality.
I personally don't use it bc I live in a fairly rural area and many back roads are higher speed with lights leading to constant adjustment.
If I'm on a really long trip I'll switch it to low manually. Otherwise I leave it in standard all the time.