Regarding feeding it rpm/deb packages, it reads the dependencies from the deb/rpm package and uses the equvalent names in shlibs (shared libraries). That’s basically a list of libs that some applications expect to find, so xbps-src just makes a symbolic link to the equvalent lib with the name that the app expects to find. Look at the example I gave above with libtiff.
Regarding everything else built from source, there are 3 types of dependencies, since the packages are built in a chroot: hostdepends - dependcies that are requires by the chroot system, makedepends - dependencies that are required to build the package, depends - dependencies that are required to run the package. The ones that are required just to run the thing are the just depends, the other 2 are required for building only.
What happens if a dependency is not available in the Void repos?
You find the equivalent lib in Void (the xtools package is a great help for a lot of things, including repackaging), add it to shlibs and that’s it. If it’s proprietery or Void doesn’t have it (higly unlikely if it’s open source… I have yest to run in a case like that), you have to put in the template as a distfile (if proprietery and only binary versions are available), or you have to compile from source (also done automatically by xbps-src once it detect there are distfiles for the lib and is not present in the repos).
Building from source is also easy in most cases (when no patches need to be applied). xbps-src has build styles (gnu-make, meson, etc.), so you just define that in the buildstyle parameter and it does everything automatically, including adding missing build dependencies.
xbps-src goes through a lot of trouble to make packaging and building as automatic as possible.