If I remember correctly it was something about Shopify discounts, which can be applied multiple times and across different modalities- percent, dollar amount, etc. and what those were called in the API, and how they were represented and to which object they were applied to.
Then once I had figured that out, then understanding how to construct the query.
But of course my problem was more from the point of view of, "I just want to get x done". As the consumer of the API I wasn't as concerned about fully understanding the entire set of abstractions and schemas.
It also means you need to be an expert in the tooling to figure it out, so just dropping in to a graphql api is so frustrating compared to plain old rest
I'm not saying the service isn't a true Scotsman, but a service that calls its API "GraphQL" but doesn't respond to introspection queries isn't really serving GraphQL.