My Macbook Pro M3 Pro does not recognize connected DSLR camera

Using the MBP built in SDXC card reader should be far faster than connecting the camera via USB for transfers, especially for a decade old camera like the D7100 which uses USB2, while your new computer has far faster USB4. Not sure if there isa backwards compatibility issue or some other problem like a cable with some hardware incompatibility. Nevertheless, I think you will be happier with taking the card out to use the SDXC reader rather than trying to get the old camera to talk to the new computer.

Hi, I too was pretty bummed out about that too, however the SDXC card slot on the MacBook Pro is beyond fast, and I believe it is a more reliable method in my humble opinion.

Oh, tethering is a different story, it's even more demanding of compatibility than file transfers. It could be that your Mac's implementation of USB protocols just does not support USB2 tethering. The Apple engineers could have dropped support for some parts of the USB2 standard in your M3 MBP. I don't know if that is the case, just guessing. But it could also be that the cables or adapters you have tried are not compatible with the Mac talking to the camera in some way. It may be time to consider upgrading to a newer Nikon, but try tethering one at a store or with a rental from some place like Lensrentals.com before you buy.