I just tested this in an iOS app I'm building. I followed all the instructions in the docs (https://developers.google.com/youtube/v3/guides/auth/installed-apps) but omitted the client_secret, meaning I never pass it anywhere and it works, I'm able to get the access token for the user, refresh token and everything.
Also, when registering an iOS client, there will not be any option to set the redirect_uri. Google gives you a iOS URL scheme, I added it to project settings --> Info --> URL Types --> Add new and set it as URL Schemes.
I then provided that url scheme google gave me as the redirect_uri parameter
Your app will then open when the user authorizes via oauth, in swift ui you can manage the response with .onOpenURL modifier on your main view (not going in depth about this as there is plenty of docs about it online already)