CREATING TECHNICAL LIBRARY

Hi Jean, I'm going to try to answer your questions from my point of view:

-Do I make separate components for desktop and mobile or a single series for both? (in the first case, should I also distinguish a mobile and destkop typography?)
I'd create a single set and then modify whenever it's necessary, to keep the library at the optimum managable level, of course there are no hard rules here, but let's say you're going to change a color a component has, editing that on a single component would reflect to all instances and states so that's why it makes more sense to me. (I'd suggest even if it's not a different component, just a state of the same component that serves as a slightly different purpose, you can include those variations in the library, showcasing the different states of the component (like default tooltip, info tooltip, warning tooltip etc, they can all be states of the same "tooltip" component right, that way library would have a cleaner "all visible" look) About typography, I can't be sure, depends on the amount of difference you need, you can even include mobile variations of typograhic elements as states of the desktop ones, again for easy managing, editing and updating.

-Is it more appropriate to use the multiples of 8 rule to develop all the logic behind the total design?

That also comes to your preference, but it sounds like it'd be helpful, I'd talk to devs that are going to code the project.

-For every modification of a component that is very different from the default version, will I therefore have to create a new component? is it normal that I will have a lot of master components?
I'd say yes, if components don't have enough similarities it's totally ok to create another component, you can use "include" logic, since a large component can include another component (like a dialogue box including a button etc, that way let's say if you change the master button component, it'll be reflected to the buttons everywhere right 😉)

Hope this helps, happy creating!