Look at the top of every forum page and you will see a link structure. Many web sites use the same type of structure. It is much like a directory structure on a computer. You can 'drill down' into the hierarchy.
The Forum Home Page is the TOP of the hierarchy. Click on it and you will be taken to the Forum Home Page. It is a page which lists a lot of statistics and things like 'Who is currently online'. If you scroll down just a bit you will find there is a list of all the individual forums and sub-forums that are here.
Admittedly this is a complex forum. The forums have increased in size and complexity over the years as visitors have requested more individual specialized forums. However, it doesn't take long to figure out.
If you *really* want a simple layout of the forum structure, just go to the Forum Discussion Thread Index
. Just look for the button in the centre of the graphic at the top left of every page. It is a simple listing of every forum and every sub-forum without any of the forum home page 'stuff'.
This forum software is vBulletin software. Unfortunately, like all software there is a learning curve involved.