Welcome! HAM (Amateur) radio is a hobby that connects people all over the world using technology that has technically existed since the late 19th and early 20th centuries. Because radios don't require any infrastructure (although systems like repeaters can be useful), radios are used by first responders, storm spotters, preppers, and hobbyists alike.
HAM licenses are issued by the FCC (Federal Communications Commission), and you earn one by taking its corresponding exam. There are many free and paid resources you can use to study for your exam, including mock exams, study guides, books, and courses. Additionally, although the question pool changes every few years, the questions in the exam are the exact same as your practice questions. After you pass your exam(s), you are issued a call-sign and given certain privileges, including being able to speak on HAM frequencies (Technician) or being able to speak on even more frequencies (General and Extra).
Once I get my license, I will put the Anki I'm using here. For now, consider the ARRL Ham Radio License Manual since all of the questions on the exam are in there. So far, I've found that the manual does not describe how to use voltmeters and ammeters well, but Khan Academy has a video with a demonstration and more in-depth explanation on how they work.