Take a look at Homebrew. Essentially how it works is that it parses a definition file for package(s) and downloads and installs them (and their dependencies) in a for loop.

You seem to be a very novice programmer.

how do I setup my dev environment ? Google that? Use a devcontainer? I mean literally a question answered thousands of times.

where should I start Setting up your dev environment. Then maybe create a few projects for you to test your package manager empirically?

do I invent my own pkg format or do I use pre built packages like deb or rpm?

Depends on what you want to do. I’m honestly not sure how to help you here? What kind of answer are you fishing for here?

Maybe spend some time investigating how to make nuget packages and then move onto deb or rpm. That way you’ll have a better idea of the problems that you’ll face in making your own. Personally whenever I feel the need for some kind of package manager, nuget is flexible enough that I can repurpose it to almost anything.