I have a laptop that is running Ubuntu. The laptop supports UEFI boot as its default. I created a bootable USB stick (using dd) with another operating system (Kali). The USB stick will boot when it is in a non-UEFI, BIOS only system and I select the USB media as the boot device. If it is in the UEFI Ubuntu laptop it will boot to the onboard hardisk even when I select the USB stick as the boot media. If I turn off UEFI and boot again and select the USB stick it boots OK. What am I not understanding about how a UEFI USB stick won’t boot on a UEFI system unless UEFI is disabled.

what is the name of the iso?

If your secure boot is enabled, Ask the internet:
what Linux distros support secure boot? (A: 2)

Instead of dd, use ventoy.net to format the USB drive. (It supports secure boot.) Then you can simply copy .iso fileS to it.

Read the LQ 'stickies' about kali
and search the kali website to see what it says about: secure boot.

I'm guessing the OP simply downloaded the one 'standard' kali install ISO. (I just took the time to verify that it won't boot if secure boot is enabled.)

P. S. Technically, there's no such thing as turning off or disabling the (whole) UEFI (as with the BIOS in old pre-UEFI computers), so further study of the concepts is needed here.

kali-linux-2021.2-live-amd64.iso

Sorry I wasn't more specific.

kali-linux-2021.2-live-amd64.iso

If I go into Setup and "turn off: Secure Boot;
Go into Advance Boot Options and "enable" Legacy Option ROM;
Then go into Boot Sequence and select Legacy instead of UEFI;

The system boots with the USB stick that I made.

I can send images if needed.

Did you check the md5sum of the iso to make sure it was a good download?

Yes I verified the check sum matched.

That is your answer right there.
dd command makes an exact clone of the iso. This tells me one thing. There isn't a fat32 partition on the iso. image.
So please boot it with legacy bios.
Or better yet at the start of your computer press either F8 or F10 I believe it will give you the options to boot from that disk/slash usb-cdrom image
slash USB-hdd image what ever the clone image was.
OK now your booted.
Look you took a cdrom image and cloned it to a USB stick. The stick will be treated as a cdrom device.
Then it will first boot from boot loader "isolinux". From there "isolinux" will then point it to a menu to boot into.
As in Slackware our isolinux setup just like many others have a efi boot image.
This tell me yours may not.

Do your have the option to boot from file in your firmware/bios settings? Maybe putting the iso onto the usb with rufus https://rufus.ie/en_US/ will work.

F12 for boot options on my laptop.

Under boot option, there is no option for CD-ROM or other USB option device other than the actual USB stick with the image and the onboard hard drive.

I did find a boot mode option that keeps UEFI boot setting but disables Secure Boot. The USB stick boots OK with that setting.

I am still researching how to make a boot stick without changing the UEFI/BIOS settings.

Modern computers use a usb as a hard drive order. It won't be a cd/usb choice.

You would need a fully supported secure boot distro if all you did was disable secure boot.