uni accommodation / firming

Hi so my school hasn’t really told us the whole university application process so if anyone could help me with what to do that would be great. So as of yesterday i’ve received all my offers and i’m confused as to what the next step is. do i have to pick my firm and insurance? and also does my firm have to have a higher grade requirement than my insurance? i’ve booked my offer holder open days so was planning to make my final decision on what to pick by then but i keep seeing people booking their accommodation on social media. do i need to do that now? can i only do that after ive chosen my firm and insurance? as i dont know what to pick im worried that if i leave it too late i wont be able to have much of a choice of accommodation.

sorry theres a lot of questions but if anyone could help explain how this whole process works that would be great )

My school suggests using the tool on UCAS where you can enter 3 grades and see the percentage of students accepted with these grades. My school has given us two sets of grades - possible and probable - and they've told us to enter our probable grades, which are lower, into all our uni offers and see what percentage of students got accepted with them. For example, my possibles are AAB and I'm going to firm Leeds which needs AAB and my probables are BBC. Leeds accepted 43% with BBC onto my course so I should pick an insurance which has accepted a higher percentage of people with BBC. I hope this makes sense!