The user _apt is created by the postinst script of the apt package (/var/lib/dpkg/info/apt.postinst):
add unprivileged user for the apt methods
adduser --force-badname --system --home /nonexistent
–no-create-home --quiet _apt || true
It’s the owner of /var/cache/apt/archives/partial and /var/lib/apt/lists/partial and used by APT to download packages, package list, and other things.
solution [FR]