samus14 to list users, you can use the built-in user list in the admin panel, the FoF User Directory extension, or the Backoffice extension.
If you want to use this information from a script of external service, the built-in REST API (GET /api/users) offers all those features. Each of the extensions above consume the same built-in API behind the scene.
There are multiple extensions that add various specific attributes to users, but for custom fields there is FoF Masquerade and Formulaire (paid).
Formulaire has an export feature built-in for custom field values together with details about the users those fields belong to. It cannot export users who have no custom fields filled in though.
