Sepheo uses Matrix for group chat, quick messages, and notifications.
Matrix is an open, secure chat platform. But instead of being a product created and and controlled by a single company, like Whatsapp or X, it is more akin to how email works: an open standard. The standard defines how clients and servers should interoperate. There are reference implementations (called Element and Synapse, respectively). But anyone is free to write an implementation of these components, therefore there are several you can choose from.
Like email, somewhat:
- First you pick your preferred client app.
- You also need to pick a Matrix server to host your account (and create it - usually the client can do that for you).
- You can then use your client to chat using your account.