It downloads (and verifies) the sources.
> What do I need to do, that the installer is using my own Xen build?
Nothing, just use qubes-builder as documented. Qubes carries a couple
local patches to Xen, so it is already built from source by necessity.
See also:
https://github.com/QubesOS/qubes-vmm-xen/
> Thanks in advance
>
> t3sserakt