Amprolla is a software written by @nextime for Devuan infrastructure. Amprolla regularly checks an ordered list of overlay repositories, starting with Devuan packages' repository, then Debian repositories, etc., and it downloads and merges all packages in a virtual
pool/ directory consists in HTTP 302 redirects to the original files or their respective servers. So, when you run
apt-get update you download the
dists/ files from Amprolla-generated repository, but when you
apt-get install something, you request an URI from Amprolla-managed Web servers that redirects to an external repository to download the .deb file. This file can either live on the actual Devuan repository (e.g.,
sysvinit-core), or the original Debian repository (e.g.,
vim, which comes directly from Debian).
Additionally, Amprolla provides "server-side pinning" to manage package dependencies with override files.
(From nextime's amprolla primer on IRC)