>
> if I understand correctly, I can install my package
> linux-lts-perso-6.1.15-2-x86_64.pkg.tar.zst, boot on it (eg. "my"
> kernel) and continue to use the binaries present on my system while they
> have not been compiled with its (new) headers and especially for the
> binaries that call the functions present in the blk-core.c file ?

The kernel headers are used to compile out-of-tree kernel modules for
that kernel. You should always install kernel and its companion headers
file. While there may be cases where the headers are not actually
changed and it may work - good policy is always install both kernel and
its companion headers package. If you don't compile out of tree kernel
modules then you may not even need to install headers.

Hope that is clear.