This is not necessarily the case at many schools.
It is a function first of enrolling in any college courses post HS (many schools will consider this student a transfer regardless whether the student attended part or full time, or how many credits the student has earned).
Next consideration is the number of college credits earned post HS, at colleges that are more lenient in their definition of a first year applicant. Some colleges have a max number of college credit hours to still be considered a first year applicant.
Some colleges require a minimum number of college credits be earned before transferring, like the UCs.
U Wisconsin is an example of a school that considers any one who has taken any college courses post-HS to be a transfer student.
IU allows one to have taken up to 12 hours of college credit post HS to still apply as a first year.