You will find dozens of websites offering a file called Xiaomi_Dex.apk or DesktopMode.apk .

If your Xiaomi lacks official PC Mode, you can simulate DeX with these apps (available on Play Store):

Xiaomi PC Mode is an built into MIUI 12.5+ and HyperOS. When you connect the phone to an external monitor/TV via USB-C to HDMI (or wirelessly via Miracast), the system offers to launch a separate desktop UI.