$ dnf download --source nautilus
$ sudo dnf builddep nautilus-46.2-1.fc40.src.rpm
$ rpm -ivh nautilus-46.2-1.fc40.src.rpm
bump the "release" from 1 to 2 in ~/rpmbuild/SPECS/nautilus.rpm
$ cd ~/rpmbuild/SOURCES
$ tar -Jxvf nautilus-46.2.tar.xz
$ cd nautilus-46.2/src
modify nautilus-application.c
  -> replace calls to g_get_home_dir with a new function g_get_starting_dir
static gchar *
g_get_starting_dir ()
{
  return g_build_filename (g_get_home_dir (), "Downloads", NULL);
}
$ cd ../../
$ rm SOURCES/nautilus-46.2.tar.xz
$ cd SOURCES
$ tar -Jcvf nautilus-46.2.tar.xz nautilus-46.2
$ cd ..
$ rpmbuild -bb SPECS/nautilus.spec
$ cd RPMS/x86_64
$ sudo rpm -Uvh nautilus-46.2-2.fc40.x86_64.rpm nautilus-extensions-46.2-2.fc40.x86_64.rpm