Zitadel could be amazing, but as far as I can tell they don’t allow using your own UI screens, and it’s not obvious to me how you’d build a multi tenant SSO feature. They have the concept of organizations, but it’s not obvious to me how you’d route a user to the right login.
You can enable Domain Discovery to route users to the correct organization. Or you send a reserved scope with the auth request to select the organization. Building an own Login UI will be available in a couple of weeks (https://github.com/zitadel/zitadel/issues/5015)
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?