So where does the money come from?
And Signal does by no means insist that they are the only ones being able to run such servers, the code is AGPL, you can take it and build your own messenger solution. They just politely ask that they keep the liberty of choosing how to run *their* instance of a messaging solution.
The Signal server can federate. They implemented that. They might not use it, but you can take it and build a different Signal with it and show that Federation…