I have a Moto Z2 Force, which has the same antenna placement. Here a brief overview.
When I’m reading my xNT (which has a similar antenna to the xDF) I position the long axis of the implant parallel to the long axis of the phone. The ideal placement seems to be the spot between the speaker and the front-facing camera light.
I’m not a medical professional, but the fatty area on the anterior face of your forearm about 10cm from your elbow seems like a good spot. There aren’t any large blood vessels, tendons, or nerve clusters there. I don’t usually bump that area, and there’s plenty of cushion. It also seems easy enough to access most readers (payment terminal or wall mounted) and you’ll probably be able to reach it with a phone.