Common App fundamentals

Common Application ( common app) is one of the popular applications used in US undergraduate admissions.
Student creates their profile and adds personal, academic & extracurricular activities in it. Student has to add their choice of colleges to the app. Note that, Not all colleges or Universities participate in common app. For those that do not participate, application is done through other apps like coalition or directly to the college site.
Student can assign recommendators to write Letters of Recommendstoins(LOR) for them. They are usually a counselor and couple of teachers. They could also add external advisors.
Student can add upto 10 activities in the Activities section. They have to also add an essay choosing one of the 7 prompts given.
Once profile is complete, student can select a college, answer any college specific extra questions, make payment and submit the application.
They will get an email from the college to setup a profile with them. All communicastion from college will be through that profile.
Some colleges need you to send test scores (SAT/ACT) directly from testing agency like Collegeboard.
Some colleges need you to self report their academic scores, some need you to get a transcript sent from school. Every college is different.
Read carefully and follow the steps that each college give.

All the best with college application.