In this case I'll be working with www-client/ungoogled-chromium available in the PF4Public overlay.
~ locate ungoogled-chromium | grep ebuild/var/db/pkg/www-client/ungoogled-chromium-107.0.5304.87_p1/ungoogled-chromium-107.0.5304.87_p1.ebuild/var/db/repos/pf4public/www-client/ungoogled-chromium/ungoogled-chromium-118.0.5993.88_p1.ebuild/var/db/repos/pf4public/www-client/ungoogled-chromium-bin/ungoogled-chromium-bin-117.0.5938.149.ebuild/var/db/repos/pf4public/www-client/ungoogled-chromium-bin/ungoogled-chromium-bin-118.0.5993.88.ebuild
I'm gonna go ahead and use the most recent version here.
~ ebuild /var/db/repos/pf4public/www-client/ungoogled-chromium/ungoogled-chromium-118.0.5993.88_p1.ebuild unpack
Chromium is huge so downloading the tar xz and then unpacking can take some time.
Once it unpacks everything you should be able to go into the work directory at
/var/tmp/portage/www-client/ungoogled-chromium-118.0.5993.88_p1/work ls -l
total 12
drwxr-xr-x 59 akujin users 4096 Oct 17 19:00 chromium-118.0.5993.88
drwxr-xr-x 2 akujin users 4096 Sep 24 04:30 chromium-patches-118
drwxr-xr-x 7 akujin users 4096 Oct 18 05:05 ungoogled-chromium-118.0.5993.88-1