How do I use Wi-Fi calling if I don’t have a carrier

Wi-Fi Calling is a feature/option offered by a carrier. Not all carriers offer this option. If they do, the option will appear when you tap Settings > Carrier.

Without a carrier, there is no way to enable Wi-Fi Calling.

There is a limited way to do it: using an app like FaceTime, WhatsApp or Signal. But you'll be limited to call to people using an iPhone (FaceTime) or using an iPhone or Android for the other 2 apps. You wouldn't be able to call a landline for instance.

This is not Wi-Fi calling per se and it requires a Wi-Fi connection.

That is simply not possible. Wifi calling is a carrier provided feature. No carrier means no access to Wifi calling.