If overtraining does exist I am pretty sure you would know. You would experience things such as chronic joint and muscles soreness plus elevated heart rate upon waking up in the morning and etc. The fitness industry is based on money. An owner of a World Gym once told me he had 3,000 members all signed up on 1 or 2 year contracts but in reality only about 300 of those members went to the gym on a regular basis. If overtraining does exist it has been blown way out of proportion by those driving the fitness industry. Convince people to train less and you can so the gyms isn’t over packed. This particular gym couldn’t handle 3,000 members all attending several times a week. The owner knows it as well.
Squatting everyday can be done and can be very beneficial depending on the circumstances. Weightlifters rely on front squats in which the lifter can’t handle the same load as a low bar back squat. I suppose if you were stupid enough to do low bar back squats everyday it might not work. Would you overtrain. I doubt it. However, you need to change the variation of the squat from high bar, to low bar, front squat, box squat, overhead squat, pause squat, high reps, low reps, and often use sub-maximal weights. For example, if you were to max out on low bar back squat everyday I don’t think you would overtrain. You would just get tired after several days and won’t have the intensity to push hard enough to challenge yourself. Rest days are a recharge for the central nervous system. Also, if you incorporated heavy/light days and used something to gauge your intensity such as Prilepin’s chart you could easily get away with performing the same exercise everyday. The advantages of this are neurological adaptation in which more and more muscle fibers are continuously recruited to fire in unison.
If you are a beginner don’t worry about overtraining at all. Just train like your life depends on it because it actually might. The thing we do today might not seem important but 20 years later in life you might be glad you put in the hard work, effort, and trained your ass off while you still had the chance to do it.
Lastly, Arnold Schwarzenegger didn’t believe in overtraining either and used high volume training. but use the term loosely saying that he always like to remain in a constant state of overtraining. His philosophy was that it was better to have done too much then to fall short of your goals. In fact, how many people do you know do too much of anything good. It is the philosophy that hard work pays off that builds a championship mentality.