I have a totally new, untouched org (Partner Developer Edition). I was going to make some customizations but before I did I created a new unmanaged package. I happen to notice though that there are already some custom fields that I didn't create (see image below). What are these? Some kind of custom field that come bundled into every org?
Yes. Depending upon type of org you signed up they come with variety of preloaded metadata(custom fields, profiles , roles etc..). Typically you go through clean up post creation of the developer org.
To give you more detailed information on types of pre-bundles metadata and how to clean them up below are some resources.
- Do all developer orgs come with a bunch of custom fields?
- Preparing and Cleaning new Salesforce Developer Orgs
Also like @David Reed mentioned in the comment you can leverage sratch orgs as they do not come with anything preloaded.
Scratch Orgs: The scratch org is a source-driven and disposable deployment of Salesforce code and metadata. A scratch org is fully configurable, allowing developers to emulate different Salesforce editions with different features and preferences. You can share the scratch org configuration file with other team members, so you all have the same basic org in which to do your development.