I am using Manjaro as well.

Are there Debian apps that you want to run but are unable to because Manjaro is Arch-based? I have read that it is not recommended to install programs compiled for Debian, that it is difficult to run them. Using a virtual machine is the recommended way to use them. Asking just in case but I do not think this is what you want.

Computers can only run one operating system at a time, unless you use virtual machines and hypervisors. Most operating systems are launched after the system uses a bootloader to get the system ready for the operating system. This is usually done by the BIOS/UEFI/firmware starting a bootloader, which then launches the operating system.

If you want a USB that you can plug into a machine that is already running, that has an active operating system like Manjaro or Windows or whatever, then have it start running Debian, like you would an Appimage or a Windows .exe program saved to a USB, that is not possible except maybe with a virtual machine program like Virtual Box or Qemu.

USB drives were not intended to be used as drives that run operating systems. It can be done, but it is not simple and can cause a lot of errors.

What do you need the USB for? If you can explain what you are trying to achieve with more detail, there might be ways to do it differently.

You could use the Debian cloud image. Just download the “no cloud” option and then grow the partitions