Serious question - how do you hold your phone? I recently moved from the 8 to the XR, and I have no trouble performing most tasks one-handed. I prefer typing with two hands, but that's a preference - I could one-hand it easy enough.
I use my pinky finger to cradle my phone (lightning port area) and my thumb has no trouble reaching the bottom of the phone, or the top for that matter. I can swipe down on the right side for control center, middle for notifications, and bottom to switch apps/access home screen easily, all one-handed with the XR just resting on my little finger. I've always rested my phone on my pinky, and moving from the 8 to the XR was more an adjustment in terms of how heavy it is than navigating the OS.