I started codeforces nearly a month ago and recently got a question: how to "actually" sort problems by difficulty?

I understand that difficulty is subjective, but for an average Codeforces user some problems of the same Codeforces difficulty still would be much much harder than others. And sorting by Codeforces difficulty and looking for the problem with average amount of solves doesn't help either, since older problems have less solves, because Codeforces was less popular. And I have some other questions:

1. How Codeforces rating system works? I heard that if the problem has, for example, 1500 rating, then you must solve it in contest if you are rated 1500 and more. But this assumption doesn't seem correct, since solving, for example, only F, rated, for example, 2600, would mean that your performance would be less than rating of problem A, which is wrong.

2. Why difficulty of the problems doesn't change over time? This would make sense since problems that might've been hard on average a few years ago, may become easier on average now, or the opposite, for some reasons.

3. Why higher and lower bounds of problem difficulty are 800 and 3500? There surely are problems that have a lower "actual" rating than 800, and I assume that there are some problems that have a higher "actual" rating than 3500.