The ones I have been to have had exactly the same sort of structure to in-person ones - you get to meet trainees, listen to grad partner speech/intro/ whatever it is, maybe do a mock case study in a breakout room and have like two breaks for tea.
I'd say just prepare as you normally would for an in-person open day. Research the firm and its main practice areas, any recent work they've been doing, what the TC is like etc. etc.
Edit: Also make sure you look alive and energised throughout the day! and try to find a neutral background as your camera will be on