what do the colleges base their admissions on,is it gpa,sat scores,letters of recommendation or essays?
In general : all of the above, plus many other things. Each college has a common data set that shows what they evaluate for admissions and what level of importance they place on each thing.
Google Common Data Set and look at section C.
legacy and race too
If you are not specifically recruited by an athletic coach, then #1 is the classes you have taken and the grades you have earned in them. Even if you are recruited, your classes and grades matter.
Everything else is further down the list, and depending on where you apply, may not matter at all.
They take each applicant’s name and put it on a peice of paper. They fold each piece of paper into a paper airplane. An admissions officer stands on one side of the room and across the room is a large basket and around the large basket is a circle drawn on the floor. The admissions officer then floats each paper airplane across the room. The ones that land in the basket get admitted, the ones that land in the circle outside the basket get waitlisted, the rest are rejected. System has worked fine for many decades, and it saves colleges a huge amount of time and money that would be wasted actually reviewing every admissions file to make what are most often arbitrary decisions anyway.
On the more serious note, what is considered varies. Many rely heavily on GPA and test score. Particularly when you get to very selective colleges, those two things lose somewhat their influence simply because the majority of applicants have excellent grades and test scores and other things in the file become important including EC’s, essays, minority status (for many colleges), things demonstrating leadership ability, and everything else in the file.