pfr There's nothing odd about it being required by other apps. That's how Android works. Apps are meant to use the media capture intents to use the system camera app to have the user capture images and videos for them instead of unnecessarily requesting invasive camera and microphone access while the app is in use.
And in many cases, other apps simply look for any app named "camera" so they will happily use the Pixel camera if installed.
No, this is completely wrong. The media capture intents can only be provided by system camera apps. Apps are not looking for camera apps to capture images and videos this way.
I generally don't give other apps camera access anyway. If I want to send a picture on Signal, I'll take a photo with my camera app and then just add it to a chat as an attachment.
The whole point of the media capture intents is not needing to give apps camera and microphone access. The whole point is that the app launches the system camera app on top of itself and you can take a picture or video with that if you want based on the app requesting it instead of granting it access. This is a very privacy friendly standard Android design that's used for the file picker, photo/video picker, contact picker and media capture intents (photo and video capture) among other things. It's not a good thing when apps require permissions instead of these things. We had to make our Contact Scopes and Storage Scopes features to work around apps not doing things in the privacy friendly way via intents. We plan to make similar features for Camera and Microphone but we'd barely need it if apps just used the media capture intents when possible.