UCAS application: I’m confused as to how this works.

I’ve applied for Art courses on UCAS for uni and all the unis have sent me a conditional offer in which i have to choose a firm and insurance choice. But some of these courses require portfolios, do i have to submit a portfolio for each uni course on the applicant portal before or after choosing a firm and insurance choice. Or do i only submit portfolios to the one I want to go to.
Writing this because i’m not seeing any portfolio submission options on the applicant portals nor have i been invited to interviews yet so i’m not sure as to how this works.