You miss he point. If you take an open source product and use cloud services to facilitate its operation, it doesn’t matter that it’s open source.
I don’t know that they are using cloud services anywhere. You can download the server and build it on your own machines. If they are using public APIs on top of that I don’t know but Telegram is itself a public API. If you want to find out if signal is using cloud services you can; can’t say the same for Telegram.