Casual question: how are the ranks assigned to each member determined

[A Canadian Cat]

I feel like I am missing something obvious but what is the algorithm used to determine the rank assigned to each member. I understand that you choose your own track and that you start at the bottom rung. I get that. I am just curious about how you get promoted along the track you chose.

Just curious and perhaps there is a page that explains it all but I could not find it.

thanks