How can I return all nodes of a certain type?

To retrieve all nodes of a certain type and expand all their fields, you can use the following query

{
  nodes(func: has(DummyType)) {
    expand(_all_)
  }
}

This query will return all nodes that have the type DummyType and expand all fields for each of these nodes. Replace DummyType with the actual type you want to query.