Hello from what I understand:
- in Bullseye, as indicated in its webpage on the Debian packages website, the linux-image-5.10.0-4-amd64 binary package is built from the linux-signed-amd64 source package
- the purpose of Debian Backports is to provide to Stable users binary packages of sources from Testing (or rarely, Unstable) that are rebuilt against stable libraries. That a binary package is present in Testing deos not implie it is (or has to be) present in Backports
- Binary packages do not have necessarily the same name as the source they are built from
:-)
