Written in 2022, 4 years ago. Tools and advice may have moved on since.
With Gatsby v4 came a much strickter approach to node creation. You must only create nodes in sourceNodes and onCreateNode.
Never in a resolver or anywhere else, even though I stumble over Gatsby Docs pointing in that direction every now and then.
And remember, if you create a node in onCreateNode make sure to add a parent, or it will be gone on subsequent builds using the Gatsby Cache.
All the best,
Queen Raae