Hi,
thank you for having an interest in OpenGeoSys.
OpenGeoSys is a simulation platform. It consists of the main binary (ogs) that handles the simulation and is accompanied by tools, which are there to help you prepare your simulation. Those tools handle tasks related to the simulation for example conversions between mesh formats (e.g.: GMSH2OGS). However, they are not mandatory to use and it is possible to have a workflow with OpenGeoSys without them.
CLI stands for command line interface. The command line is how most users interact with OpenGeoSys. DataExplorer can be used to work with meshes and output data from OpenGeoSys simulation.
If you want to fire and forget, probably the pip installation or container would be the recommended option. The pip installation has some limitations, but it would be the easiest start. On Windows, you can download the current release with bugfixes (6.4.3) with CLI and utilities. You can also find it on the release page.
It is hard to provide a single download file as OpenGeoSys is a complex piece of software. Some of its functionalities depend on external libraries, that cannot be bundled and released by us for reasons related to licensing.
I know, that the first impression for the new user may be not as good as we would like it to be. There is an ongoing effort to improve it. For now, the best starting point for you would be to explore benchmarks, find those that resemble your goal most, download them and run them. Then you can see how they are structured and start adjusting them to your needs.
The content of the project files (the ones with *.prj extension) is described in Doxygen. It is not the most intuitive to navigate through it, but you will find at least a brief description of every tag used in the project file there. There is an ongoing effort to improve the documentation for new users. You can find it here. It is a work in progress, so it is not complete and the link is temporary (it points to our development system, but I will try to keep it updated for you until new content is merged into the official webpage).
I hope, that you will find OpenGeoSys useful. If you had any problems or need help/explanation for something you encounter, please ask and the community will try to help and support you.