Realme X has industrial-grade polycarbonate back panel.
Yes, both cameras of Realme X - front & rear cameras - have HDR support.
Yes, Realme X does have face unlock support also.
No. Realme X does not have the memory card slot but you should not worry about it because it comes with 128GB internal storage option, which is pretty big from a regular usage perspective. Besides, the mobile also has OTG support.