I'm not sure why this is confusing. Unlike Android, Apple has made it easy, by adopting the standard USB-PD as their fast charging method instead of a proprietary one like Xiaomi/OPPO did.
Simply get a charger that support USB-PD. You don't even have to bother about the Wattage or anything, just make sure it explicitly stated USB-PD standard. If it doesn't say that, avoid. Simple. Most of the time, USB-PD charging is supported via a USB-C port, thus that makes your decision even easier, to just use USB-C to lightning cable. Done deal.