Can some people provide me some info and links to topics related to the general structure of modules. Is there a little doc available ?

I miss somehow diagrams and a little doc to write modules too.
Ref2read:https://xoops.org/modules/newbb/viewtopic.php?topic_id=26911&start=0#forumpost118302

Right now we are forced to study the classes in the API and to look to other modules. Two weeks of work...

If You want additional modules, I would suggest:

a) XOOPS team delivers those diagrams and a small doc, so we can concentrate to port/convert/write modules.

b) A wrapper/interface class for phpnuke & postnuke modules.

Note - Taken those links already in consideration:
<ul>
<li>http://docs.xoops.org</li>
<li>https://xoops.org/misc/api</li>
</ul>