Everyone knows the level templates in Unreal Engine when you select New Level from File menu. This is customizable which means you can add your own templates like this:
Let's see how this is possible.
NOTE: Enable Show Engine Content in Content Browser.
First create your level as usual and save it (in any name you want) Engine/Maps/Templates folder. For example, we'll save a level called My_Level. Then take a screenshot of your level and use a image editing software to make it 200x200 and import it in Engine/Maps/Templates/Thumbnails folder. For example, we'll save the thumbnail as My_Level_Thumb. Now open BaseEngine.ini from Engine/Config folder and add the below line in [/Script/UnrealEd.UnrealEdEngine] section.
+TemplateMapInfos=(ThumbnailTexture=Texture2D'/Engine/Maps/Templates/Thumbnails/My_Level_Thumb.My_Level_Thumb',Map="/Engine/Maps/Templates/My_Level")Now you'll have something like this.
Important Step: If you are on 4.25 or above you must delete Engine.ini file from Your Project/Saved/Config/Windows folder.
That's it. Open your project and select New Level.