20W charger for Non-iPhone 12
May I know if I can use a 20W charger to charge iPhone XR and iPhone 7+?
iPhone XR
Yes you may.
In fact you can safely use ANY Apple USB Power Adaptor - right up the the 96W model - to safely charge both of your phones.
(Although they will not charge your XR any faster than the 20W adaptor)
As for your 7, it won’t ever charge much faster than with the 5W “icecube” it originally shipped with.
Note that the new power adaptors will also require you to obtain use of a USB-C to Lightning cable.
More info is available here:
https://support.apple.com/en-us/HT208137