Deep breaths! Once the assessment is over you will be given contact details and there will be a follow up call/email where you can say anything you may have forgotten if need be. Stay calm
Is this the first assessment or a BOSA? In some areas the process is changing due to video calls and the ability to have several different agencies present.
I recall my first assessment took quite a while because they ask about everything going back to when he was a baby age when first words/sentences spoken, early conversational skills, physical development, coordination, eating/toileting, sensory issues, phobias, fears, quirks you noticed etc. If you still have a Red book and it's filled in properly, keep it handy but no worries if you don't.
There are a few meetings to get through and you will get this sense of "Aah what if I fuck it up" before all of them because it takes so long to get through the damn assessments!
As @Sprogonthetyne says they don't want to trip you up, they understand that you are not be an expert on ASD so you may have noticed things and not realised that it's a sign of ASD. They know how to get that info out of you, just answer as best you can.
Don't worry about rambling on either, apparently that's par for the course!