Many smartphone manufacturers choose to release their own OEM USB driver. When you have a manufacturer with an OEM USB driver, all you need to do is install that driver and it will work for your smartphone provided you have the most up-to-date version.
Doogee chooses not to release an OEM USB driver for its smartphones (at least not publicly). There are some workarounds for this if you need a driver because you want to do developing with adb debugging on a Windows computer.
The first workaround is to install the USB driver for your smartphone’s processor. The Doogee X50 comes with a MediaTek processor. Therefore, you can install the MediaTek USB driver and it will work.
Moreover, you could try the universal ADB driver. If Doogee does have USB drivers but hasn’t released them publicly, it’s possible someone has given the driver to Koush and he has included it in the universal ADB driver package.