Dynamic Desktop was introduced in macOS Mojave back in 2018. What it does is tweak your desktop wallpaper to correspond to the time of day. The same image will change to a night theme during the night time and to a day theme during the day. This is applicable only when using the desktop wallpapers included by default in your Mac. Follow the steps below to enable Dynamic Desktop:
Click the Apple logo/button on the menu bar (top left of your screen) and select System Preferences.
Select Desktop & Screen Saver.
Once the Desktop tab is selected at the top, select Desktop Pictures under Apple on the left pane.
From the collection of wallpapers on the right, select the one you like and select Dynamic from the drop-down menu next to the thumbnail of the image you selected.
There will be a dynamic change from a light to dark background wallpaper as the day progresses.