I mean nothing wrong with using a domain for routing, but I think most SaaSes would rather have a routing based on the email address.

That's basically what it does. You can activate Domain Discovery and verify a Domain on an organization, with that zitadel routes users to the organization based on the suffix (ie. email domain)

Thanks for clarifying, I must have missed it in the docs. If you see this comment, I'm wondering if this discovery functionality will also be customizable when the custom UI screens feature gets added?