I want to share the WiFi from my Raspberry Pi 3 B+ to my iPhone but I don't know how. How can I achieve this?
You have to setup an access point on the Raspberry Pi so other devices can connect to it like to any other hotspot, e.g. a home internet router. There are a lot of tutorials out there how to do it. Just google for "Raspberry Pi access point" but don't use tutorials older than two years.
A tutorial from the Raspberry Pi Foundation you can find at Setting up a Raspberry Pi as an access point in a standalone network (NAT). It uses old technologies. If you like to use more up to date networking you can look at Setting up a Raspberry Pi as an access point - the easy way.