The iPhone X has a slightly different way to power off than other iPhones
All other iPhones power off by holding down the side button - but that just starts Siri on and iPhone X
Here's how Power off you iPhone X
1. Press and hold down either the Volume Up or Volume Down button (on the left side)
2. Whilst holding down one of the volume buttons - press and hold down the Side Button (on the right side)
3. You should see a screen like this...
Slide to power off
To power on the iPhone X - just push the Side Button and wait