If ingestion from druid is done, will the metadata entered in druid be also ingested to datahub? If that's not the case, is it just fetching lists of table and column frames from druid to datahub? I'm curious about the case of superset ingestion as well.

What do you mean in the metadata entererd in druid? The datahub Druid source gets metadata from all the schemas, tables in schemas and columns in tables.