how do you charge the IPhone 12?

See the following from Buy iPhone 12 and iPhone 12 mini - Apple

As part of our efforts to reach our environmental goals, iPhone 12 and iPhone 12 mini do not include a power adapter or EarPods. Included in the box is a USB‑C to Lightning cable that supports fast charging and is compatible with USB‑C power adapters and computer ports.

We encourage you to re‑use your current USB‑A to Lightning cables, power adapters, and headphones which are compatible with these iPhone models. But if you need any new Apple power adapters or headphones, they are available for purchase.

With the supplied USB C to Lightning cable you plub the cable into an appropriate charging brick (not supplied with iPhone).

Or you use a previous charging brick and cable you have from a previous iPhone.

Or you buy a Wireless QI Charger.

Or you buy the new Apple MagSafe Charger and 20W charging brick.

That pretty much sums it up.