TL;DR there was a backdoor found in the XZ program. All major distros have been updated but it is recommended that you do a fresh install on systems that are exposed to the internet and that had the bad version of the program. Only upstream distros were affected.
This particular backdoor affects sshd on systems that use libsystemd for logging.
your Debian system is probably not affected because Debian stable doesn’t update packages very quickly. You’re probably on an older release of the backdoored package.
Nope, Debian uses a version from before the backdoor