From an external system I would only like to read all admin orders. Do I need a public, custom or private app for this? I couldn’t determine this based on the documentation (Authentication and authorization (shopify.dev)).
Yes, you would need a partners account to request the read_all_orders scope - the request is made from your partner dashboard. You can check out the process by which you request read_all_orders scope to your app here. Hope that helps!