how do they choose jersey numbers in sports?

Answer 1

Coaches especially the performance managers write up a
list. Seniority is the measure, performance, starpower all
worked into a list of who gets to pick in order. Some numbers
may be reserved from previous seasons, also up to coach.

Answer 2

well first it depends on the sport and the position. Football, offensive lineman can only have 50 through 79. Defensive players and offensive players, besides the o line, can have any number. So pretty much its the player's choice, depending on availability. And that's any sport.