On the main page there are usually 3 main sections, the bar at the top with sign in and sign out etc
The recent messages from all sections
And
The links to various sections

Click on a recent message to open that thread of messages
Or click on a section to see all the threads that relate to that section

On Phone:

The only difference is it is in a scrollable list.
First is the bar at the top, Then its the links to all the sections, but if you scroll past that you get to the «recent messages section»