What is desktop mode in Android?

Yeah, it just optimizes the screen so that the user gets a vivid view of all the data. Desktop mode has come out in only a few phones up-till now, would expect may other companies to follow suit soon.

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:

Android 10;
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.

When plugging an Android phone with this feature into a monitor via an HDMI adapter, the phone asks you to switch to desktop mode. That desktop mode is better optimized for larger displays with resizable windows and a different layout for apps

Use desktop mode to test new windowing features and capabilities added to the platform in Android 10. It's a developer option targeted for app developers and enables developers to test apps in multi-display and freeform windowing mode environments.

Regards,
J Wick