This is incoming.debian.org, a service which provides access to uploads immediately after they are accepted into the archive. It is primarily for the use of the Debian buildd-network but is publically available.

Most Debian suites have an equivalent buildd-SUITE available under the debian-buildd directory. These can be used in the normal way via apt's sources.list. For example:
deb https://incoming.debian.org/debian-buildd buildd-unstable main contrib non-free