cb-auto-actions-assigning-usergroup-based-on-cb-field-value

1/ Where is the detailed description of this plugin

There isn't really one. It's just a tool. It describes what each input does already, but how you use it is infinite so it's extremely hard to document that. Best we can do is provide some examples through tutorials and videos like we've done below.

www.joomlapolis.com/support/tutorials/107-use-cases
www.youtube.com/playlist?list=PLp0puRITgC7McQcw0Sli1qfSUQGwmflzi

mainly the list of triggers with his description ??

Below is a list of triggers, their locations, and their variables. It doesn't have all the new triggers added to CB 2.x, but it covers a good majority.

www.joomlapolis.com/support/tutorials/120-api-usage/18358-using-cb-triggers

2/ and Format Functions ??

Look at the bottom of the page. It describes what it does, how to use it, what parameters it has, and an example.

3/ and buttons:

Please read the input descriptions. They tell you what each parameter does. That parameter enables format functions on the action. They can be heavy so by default they only apply to conditionals.

Thank you for these links. I'll look at it in detail.
2/ It's just that I do not know where to
[cb:parse function="clean" method="string"]<strong>bold</strong>[/cb:parse]
in the first field? with boutton to YES?

www.joomlapolis.com/documentation/cb-auto-actions/help/auto-actions-new?pk_campaign=in-cb&pk_kwd=help
This is the most difficult to grasp tab and help is brief

2/ It's just that I do not know where to
[cb:parse function="clean" method="string"]<strong>bold</strong>[/cb:parse]
in the first field? with boutton to YES?

That format function performs cleaning actions. For example it'll clean that to a string by stripping its HTML away. You don't need to enable format functions for the action unless you intend to use them for the action. Format functions are always enabled for conditionals. Your scenario does not require format functions; they're for more advanced usages.

I learn about the immense possibilities of this plugin for a day use these advanced features.
I will wait for a detailed tutorial