Maybe stop calling it “big school” for a start! She is no longer 5.
Acknowledge her fears. Point out to her: on day 1 everyone will be nervous, absolutely everyone. No one will be bullying anyone in the first few days! It will just be about finding your way around the school, meeting classmates and learning your new timetable.
She might have the “wrong” shoes, bag, water bottle, coat and pencil case. If so and that’s hugely important then those things can be fixed with a quick shop on Vinted. No biggie.
Then ask her what’s the best way to make people like you. It’s really easy - smile and make strong eye contact and say nice and confident, “hi I’m (name).” Practice it with her. Remind her that “fake it til you make it” works - if you seem open and friendly and confident, you will make friends.
Remind her that friendships break and remake at secondary school: none of my DD’s excellent primary friends are close with my dd now. They all found new tribes as they grew up - the kpop crew, the TikTok popular girls, the sporty sensible ones, etc. Still mates but not besties.
She might be the only one without a smart phone, and that is something you’ll maybe want to think about if school says she needs a device to manage her homework.(Things may have changed but my dd would have really missed out on the class and subject WhatsApps. You do realise that you can stop her downloading apps and control pretty much everything she does if you set up the phone correctly?)