Historically, there were different naming in use for the
same architectures within different groups/projects.  For
example, amd64 architecture (Debian name) is also known as
x86_64; arm64 and aarch64; and so on.  Qemu used one way
to name the binaries, debian used another, some architectures
are named the same way, while for other architectures there's
a difference.

Starting with debian qemu-system-* packages version 8.0, we now
provide debian-compatible naming scheme for qemu-system-$arch
binaries, shipping symlinks to qemu names when qemu name is
different from debian's.  For example, qemu-system-amd64 is a
symlink to qemu-system-x86_64.

Since qemu-system emulates a CPU (it is agnostic to what
OS/kernel/abi is being actually used inside), we can use
qemu-system-${DEB_HOST_ARCH_CPU} from dpkg-architecture,
not qemu-system-${DEB_HOST_ARCH}.

In particular, the following extra names are provided:

  amd64 => x86_64
  arm64 => aarch64
  loong64 => loongarch64
  powerpc => ppc
  ppc64el => ppc64

The other names do match between qemu and debian.

There's another name which is also provided but which is
neither in debian nor in qemu: it is ppc64le, which is
the name used for this architecture in the linux kernel.

One can run qemu-system-$archos binary, and you can
list it in (Build-)Depends: field too.