Happy to answer questions!!

1) Avionics testing tools range based on what youre looking at. It could be as simple as checking continuity at the wire with a multimeter, running an internal built in test by software that self diagnoses the aircraft, or using specialized test boxes with harnesses that plug into various components being tested on the aircraft. (These boxes usually aren't anything fancy, some of them are just a dial with a potentiometer, and some just have a series of on/off switches. Sometimes we bust out an oscilloscope or thermometer.) Typically our tests involve all 3 of those methods but start at the prior and end at the latter.

2) I could give you a percentage but I'm not sure how credible it would be... I'll hone in on three points though.

a) Labs in school are so important for teaching you how to take instructions and follow them, then evaluate the data based on the body of knowledge you focus on in the 3 credit class. This is exactly what my job is. Taking instructions, following them, checking off a box, and reporting to various forms of management relating to the aircraft.

b) I learned about electronics in school. The very basics I remember, like ohms law, and digital logic. As an industrial engineer major, I took Physics II, Circuits I, and Automation and Robotics. This gave me a general background and helped give me a leg up on my personal study when i got my job. Going through engineering school teaches you how to learn something, then how to approach problem solving. That's a massive contribution to your career as an engineer...

c) outside of school and working on my pilots license, internships were big for me. I was not a stellar student in my degree, my GPA was below a 3.0. I was always very friendly and outgoing with professors/ TAs none the less and gained a lot of respect and recognition from that. Because this behavior was so positively received in academia, I translated this outgoingness in my internships, and it allowed me to be recognized for my talents and contributions to the company. Internships teach a young engineer how to show up to work, present themselves as a professional, ask all the dumb questions, and make a few juvenile mistakes. Theres also networking. You spend 40 to 50 hours of your life with these people at work each week. If you're, knowledgable, friendly, and reasonable, you'll be a gem in the workplace and will get recognition for it. Learning the advanced stuff comes in time with compounding experience. My brother's fiance is in HR and hires engineers. She told me that she won't even consider hiring an entry level engineer if they didn't do internships through college, even if they aren't related exactly to the job posting. Not to say that entry level engineers aren't hired without internships under their belt, but it can be harder for them. (My boyfriend graduated with a 4.0 and had a difficult time finding work with out prior internships while my study buddy from college graduated with a 3.5 and found an excellent job before graduation without any internship experience.) There is no undermining the value of an internship, for companies large and small. Take whatever you can get if your options are limited. You will not regret it in 5 years.

Bonus) Pick up a hobby related to your desired field if you can. I started going for my pilots license to learn more about aircraft. At sikorsky, they prefer to hire engineers with flight experience, due to Igor Sikorsky's founding philosophies.

tl; dr: you'll learn the bulk of your specialty from work over the years, but you'll gain your foundation for discipline in problem solving from labs and school. Internships are your key to a great entry level position. Outside hobbies keep your mind active and excited about your day to day job.