Reminder to self:
Debian packages are stored in library archive format.
ar -xv file.deb
This returns three files, in this specific order:
debian-binary # A small text file. Always “2.0\n” for now.
data.tar.gz # All of the filesystem bits that get deployed
control.tar.gz # control, md5sums, and pre/post scripts
Note also that data.tar can be .xz format as well.
There are dpkg-build tools for this, but all of this can be done manually for more control if desired.