The runtime wiring is used to help create the "runtime aspects" of a schema - eg data fetchers and type resolvers

It is used when you create a schema from a text file, eg from Schema Definition Language (SDL)

https://www.graphql-java.com/documentation/schema#creating-a-schema-using-the-sdl

Nominally the runtime wiring is asked via a call back "do you have a data fetcher" field this field and if so it puts it into the GraphQLCodeRegistry for you

However you can supply in a GraphQLCodeRegistry to start with and if it has the DF in it for that field than it will use it

        // if they have already wired in a fetcher - then leave it alone
        FieldCoordinates coordinates = FieldCoordinates.coordinates(parentType.getName(), fieldDefinition.getName());
        if (!buildCtx.getCodeRegistry().hasDataFetcher(coordinates)) {

The reason we have both a RuntimeWiring and a GraphQLCodeRegistry is that is possible to create a schema programmatically and not via SDL

https://www.graphql-java.com/documentation/schema#creating-a-schema-programmatically

To your specially question

Is it possible to create a GraphQLSchema from TypeDefinitionRegistry and GraphQLCodeRegistry?

not really not - the SchemaGenerator does LOTS of work to take a simple parsed TypeDefinitionRegistry and turn it into a valid schema. If you were to take a TypeDefinitionRegistry and GraphQLCodeRegistry and try to make a schema then you owuld be rewriting SchemaGenerator

I think its best to consider GraphQLCodeRegistry as a generated artifact of schema generation and not the place you "need to put DFs" upfront if you are using SDL

runtimeWiring.type("Droid", typeWiring -> typeWiring
                    .dataFetcher("friends", StarWarsData.getFriendsDataFetcher())
            )

really is the same as

FieldCoordinates coordinates = FieldCoordinates.coordinates("Droid", "friends");
coderegistryBuilder.dataFetcher(coordinates, StarWarsData.getFriendsDataFetcher()