How to use coalition app

Hi,
I am applying to colleges using the coalition app, and the essays part is stressing me out. Is it that we upload documents into lockers (which contains our essays), and send them to schools through the portal? Is it the same for supplement essays too?
An additional question (if the above is true): I checked on Stanford website, and there were 3 supplement essays I need to respond to. But when I went to the common app, there were also short questions, which Stanford don’t put up on their website. So when we use the coalition app, how do we know about those short questions? (If, by any chance, we don’t have to do that on the coalition app, isn’t that mean we are put at disadvantaged than using the common app?)