For the creation of the wicket gate, I created a commercial pivot by making an assembly (a simple thing of 3 pieces and 2 nuts). And I'd like to make it a library piece but can't select the assembly, only the components. Is it possible or do I have to reassemble it piece by piece each time in the assembly I want.
You can open your 3-piece assembly and drag it into your Design Library task pane. On the other hand, be careful, the components will always point to the initial components.
As for your parts in the assembly, are they internal or external to the assembly? Ideally, if there are only 4-5 pieces, the simple thing to do is to make them in-house, which avoids having parts files wandering around for example.
Otherwise you have to send the pieces to the library but it quickly becomes illegible.
Agree with FUZ3D but for the beginning of the design phase because to make MEPs you have to register the parts outside the assembly. For the design library, I create a folder and put my parts and assembly in it, just double-click on the folder and select the assembly to integrate into the design.