I tried to use @BenjaminFavre’s apple social identity provider solution, and seems to work fine!
I had to add a dependency to make it work (here the PR: https://github.com/BenjaminFavre/keycloak-apple-social-identity-provider/pull/1)
EDIT:
Sorry, I forgot a thing: i had also to copy manually the two html files from src/main/resources/theme-resources/resources/partials in KEYCLOAK_DIR/themes/base/admin/resources/partials directory, and merge the file src/main/resources/theme-resources/messages/admin-messages_en.properties with the keycloak file in src/main/resources/theme-resources/messages