How do Uni's know your grades if you apply without your school

One of the form tutors i am close with

SUS 😑

hahah no i i have to provide her email and then they send her a form or something to upload the reference to. Im sure she'll write something good

I'm applying for uni through UCAS but not doing it through my school as I'm taking a gap year and essentially have cut ties with the school.
I understand you enter your grades for A-Level and GCSE, but is that it? I'm aware unis can ask to see your certificates but they haven't come through the post yet, and probably won't for another month.

And I can't see how UCAS could know my grades if I haven't given them my candidate number or anything... Does the uni already have all the grades and cross-check by name?

It'd be very annoying if im denied an offer because the certificates haven't come through the post yet.

Just a bit concerned as I want to make sure everything is correct and whether or not applying without help from my school has an impact on things like this.

hi i'm in the exact same position whereby im doing the whole uni application process myself without any sort of support from my school and taking a gap year. but yesterday i got an offer from york and they've said that i need to provide certificates of my a levels but i havent gotten any because aparently they havent been sent out yet from the exam boards. idk when they'll send them out and if york will still keep my offer there if i delay my response. have you gotten any offers yet??