How do you demonstrate interest to colleges after applying?
Typically demonstrated interest is done before you apply, so once you have applied and they are reviewing your application they will see how enthusiastic you are about them, basically it will be considered as part of your application and strengthens it. This includes signing up for campus visits(or virtual visits now), going to information sessions they hold at your school, or emailing your regions admissions officer about questions you may have pertaining to the school. I'm not sure there is much you can do to demonstrate interest after submitting your application.
To keep this community safe and supportive:
Interviews, emailing for check-ups and confirmation (but be careful, don’t come across as “needy”), Online sessions, and visiting the campus or college event—just some tips, but it would’ve been best if you’ve done this prior