You mean on the HOME Menu? If you install the .cia
versions of the cores they'll show up there. Installing RetroArch from Universal-Updater doesn't install everything since that makes it super slow.
Instead you need to install Core info files
(and optionally the other entries) from the Universal-DB "RetroArch" app, then go to Settings -> Select UniStore... -> (+) -> RetroArch Cores -> retroarch.unistore and then you'll be able to download all the cores you want, just download the .cia
versions of those and it should show up on your HOME Menu.
The cores have their own separate UniStore since there's 95 of them and that would kinda clutter up the main one lol.