There is a seniority draw and pilots choose based on what's available. All those are new hire positions, but not all may be offered depending on available courses at time of joining.
The company has an option to freeze new hires, but I don't believe they have been. The freeze does not apply if you are changing bases or bidding left seat.
"Short haul for fast upgrade" isn't how it works. Upgrade is seniority based, you can bid from 777 RP to 320 CA if your seniority can hold it.
As you say positions are personal. 777RP is a good job, 10 days a month, but some guys don't like to ride in the back and want to fly so will take 320 for 15 days a month... hourly pay is the same years 1-4 for FO/RP.