Students who teach

Solution 1:

TLDR: Some of the other examples have suggested the term student teacher. In American English, this means something quite specific, and would be very confusing if you used it in other contexts. You could use student instructor instead.

In American English, a student teacher is somebody who is getting a degree in Education, probably in preparation for a career as a teacher, and who is teaching in a primary or secondary school in conjunction with their degree program as practice for this career. See Wikipedia.

Calling somebody who is getting an undergraduate or graduate degree in physics, and who teaches a class or who makes educational videos and puts them on YouTube, a student teacher would be very misleading.

If the student is actually teaching a class, you could call them a student instructor, since in American colleges and universities, instructor is often used for people teaching classes. See the Merriam-Webster dictionary.

Solution 2:

In graduate school, a student who teaches is called a teaching assistant, or "TA". From M-W:

teaching assistant: a graduate student who teaches classes at a college or university

Solution 3:

For example, there are students who teach, maybe on YouTube. Do these students, who are not yet professionals in their fields but still teach, have any name?

Yes, they are called "students" or referred to as "someone who teaches XXX on Youtube."

A: "The Battle of Hastings took place in 1066."

B: "How do you know that?"

A: "A [history] student on Youtube said it." / A: "Someone who teaches on Youtube said it."