Would recommend using an external camera to be honest.

There is a ton of software needed to get the most out of a camera, and from the little I understand about embedded image processing a lot of it happens inside proprietary blobs. You can get the image directly as an alternative, but it will look like garbage without reprocessing the input (preferably inside an open source component, with the downside of sometimes being unable to use the hardware to accelerate this)

Right now if you wanted a high quality, mostly open source Linux device with a camera, IMO you'd be looking at the Raspberry Pi, and there is still a ton of work to do. The work being done there, as well as Libcamera, the V4L2 replacement for MIPI/CSI cameras, should eventually make its way into Linux phones - but no idea when that will happen

this is possible in theory, libcamera can expose all of the bits that are needed, have fun actually finding hardware to support this though

I mean, directly "mounting" the camera to the phone and shooting with the phone.

This is pretty standard on most decent cameras, although it's usually used with the camera and phone separate. Photographers will set up a a camera on a tripod and use a phone or laptop to control it remotely. It can be used to control multiple cameras.

The youtube and tiktok generation will mount the phone to the top of the camera, usually using the flash mount, and face it forwards. This way they can see the screen while they're facing the camera, and be able to see the framing of the shot while they're shooting.

The biggest problem you'll find is that the phone apps are designed for Android and Apple, or maybe Windows Phone. I haven't used a Linux phone, so I don't know if they run their own apps, or if they run Linux programs. If they run Linux programs, then it's just a case of finding one that controls your specific camera, and has the controls that you want.

Why shoot with the phone and bother mounting it, if such a solution existed, instead of using the phone as a phone and the camera as a camera separately without mounting

Just a thought, but in a few years all old ugly photos can be refined, upscaled, content edited, rotated and animated - in 32K ultra. It could even recognize the exact mobile model a random photo was taken with and pre-set the best filters.

It prolly won't matter much if the photo is taken with a hundred year old handheld plate camera or a brand new digital mounted one - it will look great regardless.

Are you sure photo hardware is the way to go ? I think I would just use whatever you already have and upgrade the pictures later when the software allows it.