Accommodations in Bristol

Hi, I want to apply for a twin accommodation at Bristol and I was wondering if your flatmate is random or you have to find one.
I'm sorry if this is a silly question, is my first year abroad and never had an accommodation before, I appreciate your answers.
Thank you!

Personally, I'd completely at all cost avoid twin rooms

Why is that? Do people usually have bad experiences? I was thinking to try for a twin room to socialize more easily