The way Apple defines IP 68 is if it leaks, it’s your fault and you’re screwed because they won’t honor a device warranty with water damage.
Maybe it will leak (some definitely do) or maybe it won’t.
So the IP 68 rating for iPhones is only a possibility, definitely not a guarantee.
Bottom line - don’t plan on it being waterproof.
Apple is getting sued in Europe over this very issue.
Here’s what IP 68 is supposed to mean:
IP 68
(Ingress Protection)
6: Dust tight. No ingress of dust; complete protection against contact (dust tight). A vacuum must be applied. Test duration of up to 8 hours based on air flow.
8: Immersion, 1 meter or more depth. The equipment is suitable for continuous immersion in water under conditions which shall be specified by the manufacturer.