Signal's servers are hosted by Signal Messenger LLC. and funded by the Signal Foundation:

Is Signal hosted by AWS?

However, when Google realised this was happening, it opted to make internal changes that would shut down domain fronting entirely. With Google's services no longer available, Signal decided to switch to AWS for its domain fronting needs, which got picked up by Amazon through public posts.

What is Signal hosted on?

Signal and other sensitive services have used a technique called domain fronting to bypass those restrictions. Since 2016, Signal has been relying on Google App Engine to disguise its TLS handshake. The app pretends to talk with google.com even though it's actually talking with Signal's servers.