Not sure how you deduced that. Applicants are asked for their fleet preference in the HR interview. The hiring team will consider that with the applicants experience level in offering a FO position and fleet. Low experience will generally lead to a 737 offer. 1500 hours in C-5s or KC-10s and a the door to a 747 probably is still open.

Seniority and fleet vacancy, after completing a 24 month new hire seat lock, gets a FO into a different aircraft.