What does this generate?

GraphQL type definitions (type Foo { ... }) with nested types, non-null markers, and UUID fields typed as ID.

How are nulls handled?

Null fields have no ! suffix (nullable). Non-null values get the ! non-null marker.

What does this generate?: It generates GraphQL type definitions (type Foo { ... }) from a JSON sample. Each nested object becomes its own type. Field nullability is inferred from the sample values.
How are nulls handled?: A null field generates a nullable type (no ! suffix). Non-null values get the non-null marker (String!). UUIDs generate ID! type.
Can I use this as a starting point for a schema?: Yes. Copy the output and add Query/Mutation types, resolvers, and any custom scalars. The generated types capture the data shape — you wire up the business logic separately.
Does it support subscriptions or mutations?: This tool generates type definitions only — the data shape. Add Query, Mutation, and Subscription types manually around the generated types.