To get back all collections and items in the collections, you can query like this:
query {
collections {
id
title
items{
id
name
price
}
}
}
To query back one collection with all the items, then you need to structure differently.
First you need to set a query variable:
{"id": "cjwuuj5bz000i0719rrtw5gqk"}
Then you update your query code like so (notice singular collection):
query($id: ID!) {
collection(id: $id) {
id
title
items{
id
name
price
}
}
}