According to the blender manual, these are:

This is just another Blende file that some other settings are drawn from. It can have its own startup file allowing customization of the interface. It is a package of files that will contain a custom setup so that when you choose to run the application template Blender can be set up in a certain way with custom keymaps, add-ons, themes, layout and more. You can think of an application template as a set of default settings for Blender. What are application templates in Blender?

To use it, go to file -> new and choose your template. Then we open Blender and click the Blender icon next to the file menu and choose “install application template” Browse to the zip file and choose it. We save the blendfile with the name “startup.blend” put it in a folder we give the name our application template should have and zip that folder. To create the simplest application template possible, we start by customizing the Blenders interface. Thankfully, I found out about application templates. Sure, we have workspaces for this, but for anyone of those workflows more than one workspace is most likely needed. Let’s say that I use Blender for 2D drawings with Grease pencil, VSE video editing and a General 3D workflow from modeling to shading, texturing, and ending up with lighting. Ideally, we would have a system set up that would allow us to choose a workflow that we want to use and only the tools that we need would be active for that session. Since Blender is a software capable of so many things, I figured that having an interface capable of doing everything at once also clutter the experience.

Blender templates