How do medical schools differentiate between accepted and rejected students?

Medical schools will always use grades and MCAT scores as a way to limit the number of applications they look at with a very critical eye. It's important to do well in these in order to make it past the first round of application reviews that the admissions staff will do. From there, they are looking to see if you're a leader amongst your peers with high potential to succeed in a medical profession. They look to see if you are extremely invested in becoming a medical professional by looking at the type of volunteer work you do, the clubs you belong to, and if you have taken leadership roles in these types of organizations. If you do this, it will help show the admissions committee that you love medical work and will pursue your career in it diligently. Lastly, admissions committees will look extremely closely at essays you write and letters of recommendation they'll receive from your chosen recommenders. Admissions committees will look at these with a critical eye to understand what you're like as a person, and if they think you'll be a good fit at their respective school. To ensure these are of high quality, I would suggest brainstorming interesting essay topics, as well as speaking with individuals who you think are qualified to write a strong letter of recommendation. You want to make sure these individuals are as invested in writing a good letter, as you are in the application process as a whole. Best of luck, keep up the hard work, and you'll do great!