I had a virtual ASD assessment a couple of years ago, which involved two parts. I wasn't assessed for ADHD, so I can't offer any information about that process.
My first online video-link assessment was with an OT and a Speech & Language Therapist. Following on from that, it was recommended that I attend a diagnostic appointment. This was with the same two professionals, and it lasted for three hours with breaks.
This second appointment was a mixture of questions and tasks: some of the questions related to what I had written in my initial referral form, and they had also spoken with my mum prior to the appointment about my development during childhood.
Task-wise, I was given two videos to watch and they asked me questions about them. I was also asked to talk about something I'm interested in for 5 minutes. This really threw me, and I didn't think I could do it, but we had a 10 minute break from the time they told me about this to my having to do it, and they actually had to stop me from talking in the end because I really got into my topic! The hardest part for me was knowing these two people were watching me and forcing myself to look at them on the screen. Making eye contact is a struggle for me and I worried they would think that I was being rude. Of course, they weren't phased by it at all.
At the end of the appt, I was asked to stay online and wait whilst they went to discuss their notes with one another before they came back and gave me a diagnosis of ASD.
There isn't anything you can do to prepare. My advice would be to literally just be yourself, be honest and don't mask! I hope your assessment tomorrow goes well and that you get some answers.