For someone who has been in diapers their whole life, this is uncharted territory. This is mysterious stuff. They don't know where pee and poop come from or where it goes. If you take some of the mystery out of it by answering questions, that takes the anxiety out of it. Then, that can take some resistance out of it.
Basic needs are the things they worry most about.
They can't order pizza. Coming up this fall is What to Expect at Bedtime. That is a universal experience because it takes place every night, often without success. Those are questions parents often have trouble answering. I use Angus the Answer Dog as the messenger to provide answers.