Currently the ability to share with iPhones is working only on Pixel 10 series devices with StockOS.
It probably will be rolled out to other devices:
We’re looking forward to improving the experience and expanding it to more Android devices.
source: https://blog.google/products/android/quick-share-airdrop/
The feature appears to be an addition to Quick Share:
To enable, Pixel 10 owners have to go to: Settings app > tap their name at the top for Google services > All services > Privacy & security > System services. Update “Quick Share Extension” and reboot your phone.
source: https://9to5google.com/2025/11/20/android-quick-share-airdrop-pixel-10/
So if it comes to GrapheneOS, it seems to require at least Sandboxed Google Play.
Another question is also whether Google would like to use the feature exclusively to gain a unique selling point or whether it would like to roll it out to other manufacturers as well. The above-quoted phrase “...and expanding it to more Android devices” suggests the latter.