Press the wavy-lined icon on the left at the top right cluster of three icons. Or ask Siri to "Intercom <stuff>" and it'll pipe it out the speakers (and peoples phones if they set that enabled!)

Walkie Talkie has a dedicated "app" icon on WatchOS and I wonder how much more it would be used by people if iOS and iPadOS offered a more obvious link pinnable to the Home Screen like WatchOS.