When installing .deb files to install software, does its PPA also get installed into software sources?
I ask because if I find some software online which has a .deb download, I tend to look for its PPA, add it to my sources, then go through Synaptic to install the software. I do this because I want future updates for it.
But this is a longer process than installing .deb files with gdebi.
Not unless the author of the .deb has specifically added this, like Google does for their chrome .deb, it installs their repository and adds their GPG key
Other than Google Chrome I can't think of another third party .deb that does this kind of thing, PPA or otherwise, so the general answer appears to be "no".
No, the ppa is usually not installed per default when you install a DEB file using Gdebi or the Software Center.
Nevertheless your approach with looking for a ppa if there is one is a good idea as you have
Some packages however are not available through a ppa, some don't even come with a DEB file.