Do you think that it’s possible to build a web service to chat compatible with email , xmpp and matrix?
(and why not even with activityPub or rocketChat ?? )
Email is not chat (and there isn't a nice gateway for Deltachat AFAIK), otherwise yes that is possible. I run a modern IRC network that can be fully used with federated XMPP and Matrix accounts.
No, it's all automatic, but the core is IRCv3 so some knowledge of IRC is good to have for some more advanced features like channel registration and moderation.
But yes, you can create a channel though the Matrix portal and then share the link to it to IRC and XMPP users, just the syntax for these links is somewhat different for each protocol. Ideally, I could modify the XMPP invite portal (similar to matrix.to but easier to modify) to also work for Matrix and the IRC web-client, but so far they are separate.
But I think of no-technical guys and I think that create a channel and share the link is already to complex! I mean, I can teach them that, but that's not obvious as Signal or similar app.