Otherwise I LOVE the updated weather app in iOS 16 where you can see many data points in a nice graph for the next week hourly for things like humidity, wind etc.
BUT apple specifically has ExtensionKit, which is a library for arbitrary extensions in arbitrary apps. Of course, the host-app needs to support extensions and build it in. So your theorizing about a way for any app to have a plugin, its there. Many system apps in OSes have this feature, and many more apps (eg vscode) support it.
Realistically, web-apps are almost what you want too. Web-apps allow for pretty arbitrary extensions via browser plugins, which can modify the actual UI of the webpage, and make API calls to the backend (by acting as the JavaScript on the page). WebApps are basically open source though (or source available) by nature of how browsers work.
It's a really strange (and wholly unnecessary) cycle.
To be fair, this seemingly obvious feature is absent or buried in many/most weather apps and websites I've used. It's certainly not just an Apple™ simplification.
...which is a reason for people to buy a 3th party app (with a subscription + apple tax.)