In general governing bodies assign invitations according to their own ranking lists. The need to start early in case a player declines the chance. I don't think that's a process that should be public until the selections are confirmed, but the method of selection should be clear to all the players.
In Canada, we use the finishes at the Canadian Championships. So Pagulayan and Morra got the spots for invitations to the world 9b and 10b. When We have an extra slot (like when Alex is in the top 16) they go down the list of finishers, I believe.
Craziest thing I’ve read is that Dennis Orcollo has no invitation. If your invitation process excludes Dennis, it needs work, IMO.