I don’t use Instagram so can’t check the code, but I would guess they use something like

    <input type="file" accept="image/*" capture>

Which does what you’d expect on a mobile device (launch the camera) but behaves differently on desktops (opens a file upload dialog). They may have decided that this behavior would confuse users.

You can't ensure this natively either.

The photos are available as publicly accessible jpgs through http. They’re not really trying to do that.