why my iphone x shows up as apple usb multiplexor?

Most likely the iPhone driver is not installed on your PC. You need to download iTunes from https://apple.com/itunes because the one that comes built in to Windows 10 or comes from the Microsoft Store does not include the drivers.

Uninstall iTunes, then download and reinstall it. This will install the correct drivers. Be sure you do this using a Windows user account with admin privileges.

See this Apple support article—>Remove and reinstall iTunes and related software components for Windows 7 and later - Apple Support

See this for troubleshooting iTunes not recognizing the iPhone. As far as how it is being recognized by Windows, did you download iTunes from Apple, or from the Microsoft Store? The Microsoft version does not contain the drivers necessary for using iTunes. You need to download from Apple.

Have you updated iTunes lately, or did you just download it? It looks like I forgot to post the link in my first reply I told you to check. See this, If your computer doesn't recognize your iPhone, iPad, or iPod - Apple Support

Besides that, trying to get anything from the phone using File Explorer in Windows 10 is a test of your stamina. It has gotten to be very difficult, at the least, to do.

Either something is malfunctioning on your USB chain, you have a bad cable or you have a dirty/failing lightning connector.

One other possibility is you are connecting via a hub.

You aren't downloading a driver for the multiplexor, look for one for an iPhone. Or just erase iTunes completely and download it from Apple again.

See Chris' comments above regarding downloading iTunes.