deb (file format) - Wikipedia
https://en.wikipedia.org/wiki/Deb_(file_format)Prior to Debian 0.93, a package consisted of a file header and two concatenated gzip archives. Since Debian 0.93, a deb package is implemented as an ar archive. This archive contains three files in a specific order: 1. debian-binary - Contains a single line giving the package format version number. (2.0 for current versions of Debian).
DebianRepository/Format - Debian Wiki
https://wiki.debian.org/DebianRepository/FormatDebian Repository Format. This document is a work in progress that documents the structure of the official Debian repository and the format that is officially understood by clients. Its purpose is to define how a Debian repository should be structured, and how clients should interpret this. The goal is to become a sub-policy eventually.