Tennis (all sports really) is about entertainment. These days, that includes crowd involvement (don't believe me? Watch the 2013 final but without any crowd noises, or just watch any match played in 2020). So I don't have a problem at all with players who try to egg on the crowd. It builds atmosphere, so why not?
As for the ball selection, it's been a routine for, I think, decades -- not sue how much rhyme or reason there is to it, but generally players might try to look for the properties of the ball fluff and pick one that's more or less fluffy depending on what they hope for it to do in flight.
But the main reason, I suspect, is just pre-point routine. You want to focus, get back into the moment, put the previous point out of your mind, etc -- a pre-serve routine about ball selection acts as a handy "reset" in that regard, since you're doing it each point.
Also Alcaraz is cool and is the most exciting player of the next generation. But he'll (probably) lose tomorrow.