How to address students in the USA, as a university instructor: first name or last name?

First name, at every university I've been. Both for in person communication and email. There are a few charmingly old fashioned professors who use Mr/Miss Lastname, but they're seen as oddities. So yes, the communication is generally asymmetrical.

In most schools, students are called by their first name, unless there are two students with the same name in a class, or the student prefers their last name. Often, when a student is called by their last name, it means they are in trouble: "Mr. Brown, what do you have to say for yourself?"

Your own personality is a big factor in your decision.

Should you be an easy-going person for whom informality is part of your way of teaching and interacting, first names are acceptable but not required.

If there is no restriction at your University about such usage, mention your preference for informality during interactions in the first meeting with your students. "Good Morning! Welcome to Theories of General Relativity and Subatomic Particles, section IX, My name is Dr. Mason Ambicion, but you can call me Mason."

When you are teaching students who may come from other areas of the US, or from other countries in the world for that matter, standards of instructor <> student interactions will be different. Some US students will be unable to be so informal with you in a classroom situation, or in private consultation for that matter because their schooling demanded it.

When some of your students call you Professor or Dr. Ambicion, there's no need to remind them of your preference. You can try to subtly change their perspective over time by addressing them back in the same formal manner they present to you, while remaining informal with the other students.