Question

I am trying to figure out how nodes are mapped back to the fields they contain for learning purposes. How is this done?

Answer 1

In Drupal 7 you have entities and fields; fields are attached to entities. A node is an implementation of an entity (the node module implements hook_entity_info() and other such hooks) so it can have fields.

All field/entity relational data is stored in the tables field_data_field_x and field_revision_field_x or similar (the latter potentially storing revisions of field data if node revisions are enabled).

The entity_id column in those tables is the node’s ID, and the bundle is the node’s content type. The revision_id is the revision ID of the node, again only really useful if node revisions are enabled.

UPDATE

In Drupal terminology a content type is a bundle and bundles are attached to entities (in this case the node entity). When you create a new content type it gets stored in the node_type table, and when the caches are cleared (which invokes hook_entity_info on all modules) the node_entity_info() function builds up a list of bundles from the content types (have a look at the bit in that function that starts foreach (node_type_get_names() as $type => $name) {, node_type_get_names gets a list of all content types).

As discussed above fields can be attached to entities, so fields can be attached to nodes with a delta (if you like) of bundle.