Yup, dpkg-deb is all you need, but I could never find it looking through online documentation. The thing that finally helped me figure out how to make .deb files was an internet outage, believe it or not. While bored I started looking at packaging-related documentation in my computer's locally-installed manual pages, and landed on these two, which explain the basics in simple terms:

https://manpage.me/index.cgi?apropos=0&q=dpkg-deb&sektion=0&...

https://manpage.me/index.cgi?apropos=0&q=deb-control&sektion...