I heartily commend you for asking, and was happy to see you get a good response.

“What is the most common distro” is not straightforward to shat because of the breadth of users. Arch is one of the more… esoteric… distributions, it will allow you very, very fine grained control of everything - but it also requires you to be able to make those choices. At the other end of the scale we might find Ubuntu and Mint which are far less customizable* but “just work” out of the box and, as such, are obvious choices for users new to Linux or unwilling to invest in “tinkering”.

Really, the freedom of choice is overwhelming to many newcomers, and at the same time the strength of the whole system.

*Any distro is very customizable. You can make nearly the same changes to Ubuntu and Arch, it’s just that Ubuntu is not designed to make that easy for you.