In the first round, it should go to the player with the lowest initial ranking. After that, it depends, but it would ordinarily go to a low ranked player in the lowest score group.
According to rule A.2:
For pairings purposes only, the players are ranked in order of, respectively
(b) pairing numbers assigned to the players accordingly to ...
Note: Independently from the route followed, the assignment of the
pairing-allocated bye (see C.2) is part of the pairing of the last
In simple terms, this means that the bye normally goes to one of the players currently tied for last place, but there may be complications if ...