If I plug an Ethernet cable in from my Xbox One to my Xbox One X will network transfer work or does it have to go through a router?
You don’t even need to be hardlined to network transfer, you can do it over wifi.
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> You don’t even need to be hardlined to network transfer, you can do it over wifi.
Truth! But wired will always be faster than wireless 
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> If I plug an Ethernet cable in from my Xbox One to my Xbox One X will network transfer work or does it have to go through a router?
I have never tried this, but I highly doubt that direct-connecting will work. The only way I can see this working is if you give the consoles IP addresses manually, just like how you would for a Halo MCC LAN, and even that is a maybe:
Thread: Halo MCC “System Link” LAN setup tutorial videoYouTube video: Halo MCC “System Link” LAN tutorial (Direct connect two consoles with ethernet)