In order to provide a globally accessible, reliable, and high-performance communications service for the many millions of people around the world who depend on Signal, it’s necessary for Signal’s servers to be globally distributed. Having a geographically distributed network of servers is particularly important for end-to-end encrypted voice and video calls, because latency can result in audio delays or degraded video connections that quickly make the app unusable for real-time communication.

Because everything in Signal is end-to-end encrypted, we can rent server infrastructure from a variety of providers like Amazon AWS, Google Compute Engine, Microsoft Azure, and others while ensuring that your messages and calls remain private and secure. We can’t access them, and neither can the companies that provide any of the infrastructure we rent. As a small nonprofit organization, we cannot afford to purchase all of the physical computers that are necessary to support everyone who relies on Signal while also placing them in independent data centers around the world. Only a select few of the very largest companies globally are still capable of doing this, which is a hallmark of a troublingly concentrated industry.

https://signal.org/blog/signal-is-expensive/

Signal's servers are distributed globally, but yes rented primarily from American companies. I think they would diversify this if they could, but there aren't really other options with global presence. The only other host providers with comparable scale are Chinese ( like Tencent), but Signal is banned in China as of 2021 ( https://www.vice.com/en/article/signal-blocked-in-china/ ) because the PRC does not allow its citizens to have privacy from government surveillance at all.

There is no evidence that having Signal infrastructure hosted on US company servers provides any capacity for the US government to bypass Signal's encryption.