Na apt-get man page :
source
source causes apt-get to fetch source packages. APT will examine the available packages to decide which source package to fetch. It will then find and download into the current directory the
newest available version of that source package while respecting the default release, set with the option APT::Default-Release, the -t option or per package with the pkg/release syntax, if
possible.
Source packages are tracked separately from binary packages via deb-src lines in the sources.list(5) file. This means that you will need to add such a line for each repository you want to get
sources from; otherwise you will probably get either the wrong (too old/too new) source versions or none at all.
If the --compile option is specified then the package will be compiled to a binary .deb using dpkg-buildpackage for the architecture as defined by the --host-architecture option. If
--download-only is specified then the source package will not be unpacked.
A specific source version can be retrieved by postfixing the source name with an equals and then the version to fetch, similar to the mechanism used for the package files. This enables exact
matching of the source package name and version, implicitly enabling the APT::Get::Only-Source option.
Note that source packages are not installed and tracked in the dpkg database like binary packages; they are simply downloaded to the current directory, like source tarballs.
Como a última linha diz, eles são baixados para o diretório de trabalho atual. Portanto, se você abrir um terminal e estiver olhando para o seu diretório pessoal, ele será baixado lá.
Você pode criar primeiro um subdiretório para os downloads do pacote fonte, depois mudar para esse diretório e, em seguida, executar o comando e baixar o pacote fonte.