Personally I give each character a face, either from a person I know who matches their personality or a celebrity that fits the description, even though sometimes it can be really annoying if the author gives more details on the persons appearance and my character doesn’t fit that lol. I’m very curious as to what other people do.
For the main characters I usually associate them with a celebrity or fan art and I picture that while reading but typically any side characters are just a vague idea of what they look like. I get too confused if I try to come up with an exact picture of each character so I just focus on the main ones.