On the surface, the biggest difference between distros will be the package manager and the update cadence. Most package managers are generally comparable so I won’t get into that. The cadence has to do with release type - rolling or fixed - and the speed with which updates are released. Do you want the newest packages, LTS or somewhere in the middle? This is probably the first big decision to make when choosing a distro. The only real must-have here is you want a distro that provides timely security updates. Even a highly stable LTS should be pushing out security updates asap.

Then you have default package choices, which are often superficial like DE or default apps. This can all be changed so it’s not much of a concern. But there could also be more impactful choices like whether a distro uses systemd or glibc vs musl. The mainstream distros tend to use systemd and glibc, which is generally good, but know that you have other options if your specific use case requires it. There’s also package availability, meaning the number of packages available in the repository, although this is less important than it used to be because you have options like Flatpak or Nix for getting packages that aren’t in your distro’s repository.

There are also some distros created with a specific use case in mind, such as Alpine for containers or Kali for testing network security.

Finally, you have structure and governance. Some distros have corporate backing, others are community supported and still others aren’t much more than a hobby. The ones with corporate backing typically have options for paid support. In general, you want something with stable and competent governance where it will continue to thrive even as team members change. You can find examples of this in corporate-backed distros as well as community distros.

So your biggest choices are going to be cadence, structure/governance, and whether you may need paid support now or in the future.

As for what distro developers actually do… First, they build the tooling and infrastructure to make their distro work - package manager, packaging tools, repository, etc. Then, they are responsible for packaging everything available in the distro. They are pulling in source code for all these apps, compiling it and putting binaries in the repository. They rebuild packages as required when there are updates to the source code. Some distros like Arch will build vanilla packages, meaning they don’t make changes to upstream code. Others may apply their own patches for various reasons. Some like Red Hat will provide patches to upstream apps requested by customers as part of their paid support services. So let’s say something isn’t working the way you need it for some random FOSS app included with the distro. You can put in a request and they will change it for you.

As for your specific question about simulating Ubuntu on Fedora, that is not possible. They each use their own distinct package manager and repository. They generally have similar packages, but they are not interchangeable. However, there are tools like distrobox and distros like VanillaOS that have mechanisms for using another distro’s packages. These use containers under the hood so it’s not quite the same as just installing .deb on Fedora or .rpm on Ubuntu.