How to discover the IP(s) of the backend service(s) an app on my cellphone is consuming through my linux?

I want to discover the IP or URLs of the backend service(s) the food delivery app on my cellphone consumes

Make an access point on Ubuntu, have your cell phone connect to this access point, use something like wireshark or mitmproxy to sniff traffic when you use the food delivery app. You'll see both the URLs and the IP addresses.

You can block either by configuring the DNS services on your router (best way, if your router can do that, or if you install something like OpenWRT where you can do that), or you can block the IP addresses. But be aware that if the app uses some cloud bases services, the IP addresses will likely be shared by lots of other services, and they also can change randomly, so this is an unreliable method.

I don't have much knowledge in networks

Then expect to spend quite a bit of time in experiments and reading network stuff until you can make the above work.