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.
To add more possibilities/perspectives to the above:
The security question I’ve seen most in my life has probably been “What is your mothers maiden name?” which becomes fairly easy to guess with family history.
Ancestry information can reveal who is inbred.
It also can reveal politicians commiting nepotism.
Geographic location can show if someone lives in a redlined neighborhood or the part of town with all the mansions.
Simply the fact that an account exists on 23andme’s website, implies someone took the test, which indicates they (or someone they know) has disposable income. Enough to pay for such a test (initially I believe it was $400 but I could be wrong) and that also implies they have some form of internet access and that they probably own a smartphone/computer/laptop/some kind of technology they can use to access their account. Thus they could be targeted simply for having potential income/assets above that of poverty level.
If actual DNA data was comprimised, which I doubt happened but suppose it did, an advanced enough attacker could use that to plant evidence at a crime scene. Who would believe a whistleblower after their DNA was found on a rape victim? Who would vote for a politician whose DNA was found on a murder weapon used to kill dozens of missing persons? They can scream “fake news!” all they want to, once that seed of doubt has been planted, once enough people are made to believe someone is guilty of some atrocity, it is hard to shake that belief. The DNA evidence is there. It was tested by scientists.
I could come up with more far fetched scenarios too. I made a list of them once because a family member purchased one of the 23andme tests for me to take. They did not understand why I refused to take the test. The reason was because a decade and a half prior, I was charged with a crime. The crime doesn’t exist anymore where I live (illegal botany) but at the time it could have been a felony. I did not want to have a felony. Felons had their DNA added to a federal database to assist investigators in finding repeat offenders. I fought hard to ensure I was not convicted with a felony and succeeded by pleading to lesser charges.
The idea of having my DNA on file with a government agency like the FBI, CIA or NSA terrifies me. A malicious agent could do a lot of damage with it. They could invent threats with it to ensure I comply with their demands. The amount of possible damage they could inflict grows every day with new technology. DNA, gait and facial recognition, geofence data and an AI trained to make deepfakes, in the hands of a shadowy alphabet agency with little oversight, that’s fairly unstoppable by a single person. Imagine if anyone could get their hands on that. A disgruntled coworker. An obsessive ex. A hormonal teen child having a temper tantrum.
I know this is long and extreme in parts. I hope this helps people understand that DNA data is powerful and could be abused in unimaginable ways.