Initial interview was done using HireVue, asking for code examples in Verilog, projects done in the past and the standard 'who are ARM and why are we great?' questions. From hearing from other applicants, this is the exact same set of questions for everyone, so don't expect a personalised interest into anything you've done.
After this I heard nothing for a month until (with under two weeks notice) I was invited to a full day assessment in Cambridge. Be aware that ARM has a strict policy that if you're under 3 hours from Cambridge, they won't compensate you for overnight accommodation because of how popular they are, as it was explained to me.
This popularity was apparent when all 7 of us turned up for the assessment day. The day was mostly spent doing team-building exercises (stop an egg from breaking when dropped, make a presentation on things you'd bring to a desert island) while quite unnervingly surrounded by completely silent observers making notes on clip-boards. This is interrupted by short breaks to speak to recent graduate employees who turned up for free food, and a brief technical interview.