In my case, I need to make an in-house communication system using Apple watches (without iPhones) There I'm planning to use UDP or TCP protocol for voice communication. And is it can able to receive a notification without iPhone?
watch OS 6 and above now allows independent Watch apps.
Check out it's WWDC: Creating Independent Watch Apps
watchOS 6 enables a whole new level of watchOS experiences by allowing fully independent apps and apps built just for Apple Watch, and by bringing the App Store to Apple Watch. Discover how to leverage the power of many iOS frameworks and technologies, now on watchOS, to create fully independent experiences on Apple Watch.
For lower versions, an Apple Watch app installs as an extension to an iPhone app. You would need an iPhone to access the AppStore to install an iPhone app, which provides an extension app to the Apple Watch.
In this case, you can create a simply, bare minimum iPhone app but with a complex Apple Watch app extension.