I would like a detailed outlook of how they select students
Any colleges in particular? Have you visited any schools websites to check for application requirements? You can also google common dataset information which shows what the schools are important in their applicant review.
There are 3000 colleges in the US and 3000 answers to your question. You need to do some research on your own first.
@TomSrOfBoston is correct. There is absolutely no one answer, no one magic formula that fits every college every person, every situation. Each college has information on the Admissions page of the webste which should be read carefully (ex. recommended coursework, class profile etc.) The most helpful general advice is to be the best “you” you can be in every sense (academics, ECs, relationships with teachers etc.) and do each application with care (ex. attention to essays, supplements etc.).
If looking a a LAC maybe read “The Gatekeepers” (it is an older book but was an interesting read) https://www.amazon.com/dp/0142003085/ref=rdr_ext_tmb
So I decide to order through inter library loan The Gatekeepers from the College where I work knowing that it might come from my son’s high school since I saw they had a copy. Sure enough it was and then I noticed oddly that the author had signed the copy. As I begin to read the book, I find that he hails from my area. A little more digging and I find that he actually graduated from our high school!
Here’s some background on the selective private college process:
http://talk.collegeconfidential.com/discussion/comment/17885954#Comment_17885954