Google Assistant can control some aspects of your phone by voice alone. The trick, like in setting your phone’s maximum or minimum brightness, is knowing the right thing to say.
You would expect that if you said, “OK, Google, set brightness to maximum” or “set brightness to 100” it would work as you might expect. That is, it would crank up the brightness to full. It doesn’t always work that way.
Android Users have reported that asking Google Assistant to set the brightness to maximum or minimum doesn’t always work. Others say it does, I have confirmed for the Google Pixel XL it certainly does work. You can also try saying set brightness all the way up or down which works as well. It may depend on your manufacturer settings.