The sounds you refer to are called, imaginatively enough, "pick noise", something most people don't like to hear. In many cases, players are not aware of it while they do it (busy doing the playing, and not listening very carefully), but in recordings it can be quite audible/annoying. As with any sound, a person may or may not like it or even notice it. But it's generally thought of as a bad thing, and an obstacle to clear and clean playing. The records you've listened to probably don't have much of it, or any, to start with due to good playing technique, but if the raw recordings have it, it can be mostly eliminated with careful equalization in the mixing process.
There are various theories of how to eliminate it in playing. They involve hand position, pick stroke, how the pick is worn, etc.
My own theory is that when the pick moves quickly enough through the string, pick noise tends to go away. I associate this quickness with having the right hand "warmed up". Slower pick motion perhaps allows the vibrating string essentially to clatter against the pick momentarily just before the string is picked.
In reality, I have advised players that when they notice pick noise, they should focus on it and without any specific change of technique, concentrate on "making it go away". If you dislike it enough, it will actually tend to go away as you do this. I think in fact the consciousness of it, and disliking of it, prompts the hand to make small changes which at some point result in less pick noise. If a person is focused on not wanting it to occur, those changes tend to become locked in.
I know this sounds pretty inexact and even mystical, but it does work for me. Some years ago, Bela Fleck and I did a workshop together at a festival. Someone asked about how to deal with pick noise. I asked Bela to answer first. He began by saying, "Well, first of all, you have to hate it," and then in his own way described what I just said above. I felt vindicated in my odd-sounding idea, because Bela is a very diligent student of all things banjo.