The “new field dropdown” list contents are populated in $:/core/ui/EditTemplate/fields.
If you search that tiddler’s source for <<qualify "$:/state/popup/field-dropdown">>, you will see that the code that follows it actually outputs the list contents.
The “user” field items are produced by this $list filter:
all fields that are defined in “regular” (non-shadow, non-system) tiddlers, excluding created, creator, draft.of, draft.title, modified, modifier, tags, text, title, and type.
and system fields are:
all fields that are defined in any tiddlers, excluding any fields defined in regular (non-shadow, non-system) tiddlers.