In 2023 three products had three login screens and two payment integrations. Every VAT change cost us three times the work. That got old fast.
Simezu stores who someone is, what they pay for, and on which devices that session is valid. Apps ask one endpoint and get an answer they do not have to interpret themselves. Modular and Rozuro connect through an extension. The rest talks to it directly.
Under a client brand it is called Kimezu. Same code, different name. That is the whole trick.
Simezu is not an identity provider for the rest of the internet. No SSO federation, no SCIM. If you need that, use Keycloak. We built the layer we needed under our own products, and left the rest alone.