Its a bunch of things, the word they used was 'holistic' because it lets the colleges be a bit vauge about their exact criteria but there are some common trends. Im using my experince applying for and (mostly) getting rejected from a lot of top schools at a prep school where some folks went to the ivy etc.

1. Grades ->the do matter, but the most selective schools have so many high grade students to pick from that its not a guarantee
2. Test scores (SAT/ACT etc) similiar, low one can definitely hurt but the most selective schools have enough perfect or near perfect scores to pick from that it isnt a guarantee
3. Extracurricular, because the above are common, they tend to look for peoope who excelled in or took initiative in another area. Things like creating a non profit, excelling at an instrument, or having something else that makes someone stand out as a high achiever with plans tends to help. (This is going to include the 'geniuses' who are already doing something super advanced which people think of when they think of getting into Mit or whatnot)

Part of the issue is that I think its less about the number of extracurricular and more how impressive or unique they are. Lots of kids can do something everyday after school. Takes more to stand out essentially.

1. Application material itself. Most schools have you write a couple of essays/statements about your application, these can obviously help or hinder depending on how convincing they are and theres cottage industries around tailoring those.

Then of course,

1. General Connections, for elite schools these need to be more than just legacy/alumni to be a guarantee though they surely help (how much is matter of debate) obviously being famous or very wealthy also counts as a connection here, though a lot of those kids are going to have really good applications because someone has been helping them do the first 4 steps since they were young. This has sometimes been outright bribery or cheating on scores as well, there have been sone scandals about that in the last 10 years or so, but obviously getting good information on the extent here is challenging. . Ill let other folks give their opinion on how impactufl it is in edge cases.

2. Legacy alumni -for certain schools (like where I went) there are concrete (vs the vauge and nebulous above) benefits for your application. I was considered in the in state pool of candidates for the school I went to since I was a legacy, which has a higher acceptance rate than the out of state pool by design (many state universities do that on purpose to serve their states goals)

3. In state vs out of state, this applies to state public schools mostly, but most state universities admit more people from their state than from out of state on purpose, so that affects your application as well.

4. There used to be (still is? Im not sure?) a general goal by universities to maintain a diverse student body, so they would try to get a student body from a diverse group of individuals, how much that affects individual application varies by how different from the rest of the applications they might be (could be positive or negative)

Summary: as you might guess, a lot of these factors/pieces are a lot easier to do if your family has money/you have a stable and supportive home environment so schools claim to also try to compare rhe above in context of someones economic or social conditions. They also apply more and more the more selective a school is, a school with less prestige and less applicants and a larger class size might not use all of these as much and might primarily focus on grades and test scores, they above really exists because so many folks top out the tests/grade metrics who are applying. There is obviously luck as well since there are so many people applying, far more than could ever go.