Thank you, Ciarán!
I managed to get that far, by using Custom HTML modules. The problem that I am seeing is demonstrated in the attached image file. When the home page is displayed "fresh", the content of ALL the modules is displayed, as if in a blog... Once I click on any of the tabs (e.g., "1ST TAB"

, all works well. If I "go home" by clicking a Home menu item or the logo, the problem returns.
Would you recommend that I use something other than Custom HTML (e.g., article in a module extension)? If yes and you can recommend something (especially from Joomla51) please tell...
As you can see, everything is being put together as we speak so I'd rather not share a link publicly (or admin access, even privately). I hope this is enough for you to give me a direction. If not, I will keep banging on it and if (or when) I accept failure, I will provide link/access.
Many thanks,
-g