It's definitely worth calling them to ask what the assessment actually means @pigletpie2177 (and welcome to the thread!)
We had several "assessments" before we got to the final diagnostic one, so I'll tell you about both (although DD started the assessments at around 2 and got her Dx at 4, so she was younger than your DS.
In the earlier assessments we were generally all in one room with a clinical psychologist and at least one other professional (e.g. a trainee CP or SLT) and they would play with DD a bit and then the junior professional would continue to play with DD while the senior person asked us lots of questions about DD's development. So it's worth brining anything you have like nursery feedback/reports, your red book if you made notes there about when DS hit various milestones, any other notes or e-mails you have where you've talked about where DS is developmentally at various points. Then for the final assessment they didn't need to take a developmental history from us (as they'd got it all from those previous meetings), although I did send them some written notes to update them/fill in gaps. But obviously if you were only having a single assessment this developmental history would be part of it.
In addition to the developmental history they went to visit DD in nursery (I think it was an SLT who went to do that part of the assessment), and then did the ADOS at the clinic. That's a play-based assessment - just to see how DS plays and interacts with adults. We were allowed to be in the room with DD but she wasn't quite 4 at that point - although I don't really see there's any harm in a parent being in the room so hopefully that would be possible for DS too if he felt more comfortable that way.
In my view it's worth also starting to talk to your DS about it (does he understand a bit what's going on?) At that age it could be more generally about different brains working differently and different people playing in different ways (DD wasn't interested in the idea of autism until she was 7, but we'd always talked about different people's brains working differently as a reason why she was sensitive to sound, or struggled with dealing with lots of other kids at once, or was able to super-focus on one topic and become a real expert on it, for example - so by the time we gave that a "name"/"label" it was not too much of a shock).
And yes to asking them about support. We were just thrust a massive wodge of leaflets but most of it didn't seem relevant and I didn't really know how to navigate it. The best thing I'm on is a local SEN parents' WhatsApp group which is really helpful for finding out about what is going on locally and how to access local services, deal with problems at school, etc. etc. It might be worth looking for SEN parents groups where you live (e.g. on Facebook, or googling your local authority name and "Local Offer" and seeing if there are any parent support groups listed there). I remember feeling awkward about accessing those spaces pre-diagnosis, but actually in some ways that's when you need them most!
And yes definitely leave time for cake or something else nice after!! It is intense and can be upsetting. It kind of forces you to dwell in detail on negatives, so you need a bit of time to shift your brain back to positives and optimism afterwards!