Learn Hardcore Russian: Телефон заряжается, пока я читаю книгу. - The phone is charging while I am reading a book.

Right now / currently: The phone is charging while I’m reading (at this moment).
General habit: The phone charges while I read (e.g., that’s what you usually do).

Because пока introduces a subordinate clause (пока я читаю книгу = while I’m reading a book). In Russian, a subordinate clause is normally separated from the main clause by a comma, so:

Main clause: Телефон заряжается
Subordinate clause: пока я читаю книгу

What does пока mean here, and how is it different from пока не?

Here пока means while / as long as, describing two actions happening at the same time:

Телефон заряжается (the phone is charging)
пока я читаю (while I read / am reading)

пока не usually means until (often with a completed result in mind), e.g.:

Я подожду, пока телефон не зарядится. = I’ll wait until the phone charges (finishes charging).

Why is заряжается reflexive (-ся)? Is the phone “charging itself”?