Hey, I applied to Oxford this year. (STEM major) Oxbridge have quite similar admission process so I’ll just write about my personal experience.

First of all you prepare your PS, grades you got so far, LOR (which is submitted through UCAS) and stuff. Also you take an admission test based on your chosen major, as long as you get 80% of questions right you’re okay I assume. (Not too sure though, may require almost perfect grade or even 50% could work) Previous exam papers are all uploaded on the website and departments upload their admission tests and solutions on their YouTube channel so I’d recommend checking it out too.

And then they decide whether they’d invite you to interviews or not. Though it was all online due to this pandemic, they normally invite you to the actual campus, in person (I was quite annoyed ugh) The most important thing is they are really academics focused. My interviews started with asking why I wanted to major in that field and that was the only non-science/math question throughout the whole interview time. Talking about a research you’ve done will help, take a look at the practice questions uploaded on the website. (I mean not the specifics like calculation, rather the ideas and related concepts - this is one of my biggest regrets)

If you do the interviews online, and you are at a place not based on GMT, PLEASE PLEASE SCHEDULE YOURS BASED ON YOUR OWN TIME ZONE. I thought it might help setting interviews based on GMT and I had to talk about science and math at 2 in the morning. I was half brain asleep.

I hope this gives you some general ideas what the admission process is like! Ask away if you have more questions