While there are a lot of ways and tools to write bootable ISOs to USB on a Mac, the most easiest is in fact by using the "linux" method, a perfectly valid option using internal command-line tools:

You'll need an USB device that's large enough. For netinst, 1 GB would be sufficient. Make sure that the contents are kept safe if needed, as the USB device will be wiped.

If you haven't done so already, you can download the proper ISO file for Debian from this page: https://www.debian.org/distrib/. Download only from a legitimate mirror.

Insert the USB device and open up Terminal. We need to verify the device identifier of the USB device as we will need this later. This will be in the  ...