HDMI ports are unidirectional, NOT bidirectional. You cannot plug a camera into an HDMI port designed for video OUT, such as on a GPU or laptop, and get video from it. You MUST have a capture card/device, like an Elgato Camlink. Some laptops DO provide an HDMI IN port, but it is only to allow using their display like a monitor; the video signal is never captured by or accessible to the laptop itself, it's just allowing something else to "steal" the display panel.
DSLRs cannot be used as webcams over USB. Canon and Nikon may provide preview software, but generally it is low-quality and low framerate. You'll have to capture their preview software, as it does not present as a UVC webcam. Better yet, again, get a capture device. But check to make sure that your camera provides a 'clean' HDMI output, and doesn't include the onscreen UI (many Canon and Nikon cameras do this).