Yes, Realme X does have the in-display fingerprint scanner.
Yes, Realme X does have the support for OTG.
Yes, Realme X does have the Dolby ATMOS support. It is enabled by default.
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.