Since colleges are going test-optional for the class of 2021, what will they look at to choose the students they want?

Despite being test-optional, colleges will still consider your SAT score if you choose to admit it. A high score can still increase your chances of being accepted. Most colleges take a holistic approach, meaning they look at every aspect of your application to decide if they think you would be a good fit there. Your test scores (if you submit them), your GPA, the rigor of your classes, your extracurriculars, your personal essay and who you are as a person, your supplemental essays, they all impact the college's decision.

As ajdekonik said they still consider it but some have test flexible paths usually substituting an interview for SAT score ex Drake. But SupertutorTV on Youtube predicts essays are going to be way more important assuming a holistic admission procress but as you are taking the SAT it still is a factor assuming you submit the score as test optional largely sees SAT like a recommended letter of recommendation really only can boost you except if you get a 1000 SAT and want to attend a top 50 school.