Hey Squirrel, notifications do work across profiles if you are logged in with a session on the other profile, too. This will work until you restart your phone or explicitly end your session on the other profile otherwise you will stay logged in when switching profiles. There is a setting in graphene under System > Users > Send notifications to current profile to toggle this feature on but only the profile name, app name and time is displayed so if you wanted to see the message from WhatsApp then you won't be able to if it's in another profile.

I decided instead to put WhatsApp into a Private Space in my owner profile by itself so it's isolated, but I can access and use it on my main profile normally, only that the private space needs to be unlocked when I restart my phone.

Personally I don't have WhatsApp notifications turned on, only when I recive voice or video calls from WhatsApp then my phone will vibrate, but you can get notifications properly with this setup if you like.

You can read about how the private space feature works with GrapheneOS here: https://discuss.grapheneos.org/d/16670-private-space-on-android-15-grapheneos

You will find, as mentioned all over GrapheneOS forums, that there are millions of ways to setup profiles and lots of discussions around it so you just gotta work out what works best for you.

My setup is kind of a half way house to full degoogling.

1. Owner profile: mostly FOSS with Fdroid, Obtanium, sandboxed Google play with a new burner Google account for the few things that needed it.
2. Owner's Private profile: WhatsApp
3. Secondary profile: banking apps
4. Google profile: my left over Google services. Profile will soon be deleted entirely.

Sorry for the long reply, hope it is helpful.

That's the neat part. You don't!

Seriously though, if its in a separate user profile or a work profile you won't get any notifications unless it's active. The simplest solution is to use something like Shelter to get create a work profile. Put G services in that and WhatsApp and anything else. Then you can pause the whole profile when you don't want it running, and G can only see what's in that profile. This is what I do and I keep it paused 99% of the time.