In short, the desktop mode in Android 10, by analogy with the Samsung DeX application, allows you to connect an external display to an Android smartphone or tablet to make it easier to view content or work with a connected keyboard and mouse (like on a computer).
At the same time, one application can be displayed on both screens, as well as two different ones without stopping work (using the multitasking recovery function). Plus, the ability to move and scale windows on a large screen, the ability to simultaneously use the on-screen (virtual) keyboard on the smartphone/ tablet screen as a computer keyboard, etc.
How to enable and configure Desktop Mode / Desktop Mode in a smartphone with Android 10
So, in order. Of the most essential requirements:
DisplayPort Alt Mode support (it is supported by most models with Qualcomm Snapdragon 835, Qualcomm Snapdragon 845, Qualcomm Snapdragon 855 processors, equipped with a USB 3.1 Type-C port, but the full list of devices that have a video output function via USB-C can be found here );
adapter from USB 3.1 Type-C to HDMI;
monitor / TV with HDMI-input;
Bluetooth mouse and keyboard.
If all of the above is available, then further:
open the "For developers" section in the smartphone settings;
Scroll the "For Developers" screen to the "Applications" section and activate the "Enable freeform windows" and "Force desktop mode" options:
reboot the smartphone;
after that, download and install the customized version of the Lawnchair application from the project page on GitHub (LINK to the APK file);
after the installation is complete, tap "Settings" -> "Applications and notifications" -> "Default applications" and set Lawnchair as the main application (the default launcher);
we connect the smartphone to a monitor or TV through an adapter;
give the application the necessary permissions (display on top of all windows, etc.) and, in fact, test the Desktop mode on the big screen.