You want option C….
The reference to wifi updates, refers only to the updating of the generic Garmin maps installed on the device. I do not know of anyone ever managing to install third party maps via wifi, though there are some clever bods who might know how to do it. Assuming that is, that it’s even possible.
As to the rest?
You can install maps onto a Garmin device, using Garmin’s Map Installer app.
It is though maybe easiest to put the third party maps onto an SD card, which is what I sometimes do if I want to run some Open Street Maps on my Garmin device.
https://support.garmin.com/en-US/?faq=21G3VnXffc3fvlt4Ru9Yg7
There are lots of easy to follow instructions on the internet, each telling how to do it all.
One tip: If the Toppo maps are routable, then you’ll be able to create, display and run a route via your XT’s screen. If they are not, then you won’t. I am guessing that they are.
Second tip: You cannot run Garmin and third party maps simultaneously. When you come to run the Toppo maps, remember to uncheck the generic Garmin maps and to check (tick) the Toppo maps.
Third tip: Remember to re-check the Garmin maps and uncheck the Toppo maps, when you are done.