Does SLIM Companion Explorer automatically extract email metadata from emails?

Yes, the email metadata such as To, From and Subject are automatically extracted from msg and eml files. The values are captured in the corresponding SharePoint / OneDrive for Business columns.

See https://slimapplications.zendesk.com/hc/en-us/articles/207876245-Which-columns-do-I-need-to-capture-the-email-metadata-fields- for a list with supported properties.