I think self hosting this kinda thing is a pretty rough first project. At the moment you would also have to host your own matrix server to make this work.

This is the code for hosting the bridge: github.com/mautrix/whatsapp

There are also ones for signal, telegram, etc matrix.org/ecosystem/bridges/

The EU and the matrix foundation do a lot to push changes to make this interoperability more user friendly while maintaining E2EE, but its just not there yet.

At some point the messages just need to be decrypted because the encryption algorithms all these apps use are different. So unless you directly add whatsapps or signals code into your app, this will have to happen elsewhere, like your or someone elses homeserver. If you dont care about that or if you trust that server, you can try to find a server that has an existing bridge running.

tchncs.de for example is running a signal bridge tchncs.de/en/matrix/ (scroll down all the way)