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 format of /dev/disk#, where # is a number. It is important to correctly verify the number as selecting the wrong one will effectively wipe the wrong disk (possibly some other or even your internal drive).

diskutil list
.
.
.
/dev/disk2 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
.
.
.

There will be more disks in there, but the one stating "external, physical" is your most obvious candidate. But again, please verify and double check. If you're unsure, ask someone to verify it for you.

If the USB device has one or more mountable partitions on it, first unmount all:

diskutil unmountDisk /dev/disk2

Next, run the dd command to write the ISO file to the USB device. Do not convert it as stated by some other tutorials: this is not necessary.

sudo dd if=/Users/lunarg/Downloads/debian-12.6.0-amd64-netinst.iso of=/dev/disk2 bs=1M

Writing the ISO may take some time on slower USB media (pre-USB 3.0), and you won't see a progress bar of any kind. When it's finished, you'll see output similar to:

631+0 records in
631+0 records out
661651456 bytes transferred in 86.324320 secs (7664717 bytes/sec)

Note that you will also get a pop-up like this one: click "Ignore". It is normal that the USB device is now unreadable by your Mac:

Remove the USB device and insert it into the computer on which you want to boot/install Debian from. Set it to boot from the USB device.