I'd suggest picking a package and follow it's creation in Ubuntu, through to upstream. I'll use gcc-10
(https://packages.ubuntu.com/focal/gcc-10) as it was the last package I looked up. To the right of the details on the package you'll see homepage (upstream clues), maintainers, changelog & more. Compilations are done by infrastructure/builders (machines not people)...
Compilations are done locally; then uploaded often to PPAs for testing by ourselves, others in teams before committing to source for upload to Ubuntu etc.
Useful reference (but there are many more!)
but many guides exist, eg. Lubuntu's can be found at https://phab.lubuntu.me/w/packaging/
In comment you asked about paraview
A link may have been useful (https://packages.ubuntu.com/impish/paraview) shows me Ubuntu Developers maintain the package as you said; at the top I see it's a universe package (ie. community support only) with the upstream developers Debian Science Team (https://packages.debian.org/bookworm/paraview) so for example I maynext look up https://tracker.debian.org/pkg/paraview for more clues IF the Ubuntu changelog wasn't enough (https://changelogs.ubuntu.com/changelogs/pool/universe/p/paraview/paraview_5.9.0-2ubuntu1/changelog; look at changes; names/emails etc).
Debian is upstream, ie. the source.
A large percentage of Ubuntu packages come from upstream. I'm involved with Lubuntu (thus my providing that link as I knew it earlier); our LXQt we provide has been newer in Ubuntu than Debian has for last few releases, as Debian were often in freezes & it was easier to package only for Ubuntu/Lubuntu. Debian isn't in freeze anymore so the Lubuntu devs/packagers were discussing only recently packaging the next version upstream in Debian, so it'll flow back to us (thus Debian benefit from the packaging too... Many devs (but not all) have @debian and @ubuntu emails.. you'll discover, so don't be surprised if the one name appears using both!)