Phone screen was fine, 1:1 via video with a member of the design team was great, so I thought the onsite would be great as well - I was wrong.
They have a "share your portfolio" session onsite before 1:1s start and no one will tell you but be warned it is a tense and formal presentation and the group asks you questions after. Be prepared to immediately present to 8 total strangers who may, as one female designer did to me, roll their eyes at you while you're speaking.
I don't think they look at your portfolio at all before the onsite, if they had, I would not have had to spend an uncomfortable hour onsite with them.