In what table does an entity "foo" get its metadata stored? In Drupal 7, there are tables for all the fields for a content type and the node table had some metadata stored, but most data for a content type were in other tables. I think I understand that configuration is what metadata is required for an entity. But I cannot figure out which tables would store an association between node/100 and user/10. Using breakpoints I'm trying to troubleshoot where in the node object this info resides and try to deduce its definitions to backtrack where metadata like author/owner is stored.
In Drupal 8 all that stuff is stored as YAML files in your config directory.
If you don't have a custom path to that directory it's usually located in
sites/default/files/config...... where .. is a long randomly generated hash key.
There you will find config files for all entities and fields and configuration settings.
EDIT: To get a node's asociation with it's author you'll have to look in the
node_field_data table, the UID field there is the author's user ID.