What if a would like to export all group libraries?

unfortunately, yes. For >5000 references shared in a group I found it suitable to have >10 group libraries for different topics.

It might be wrong, thus you could correct my intention. I've decided to divide the collection, because originally it took too long to proceed, i.e. show all the references stored in a single collection.

Could you confirm that if I put all articles into one group folder and divide them into subcollections, Zotero would easily and quickly open these subcollections.

Could you confirm that if I put all articles into one group folder and divide them into subcollections, Zotero would easily and quickly open these subcollections.

but the answer to that question is yes - though with 5000 item you shouldn't see much of a delay either way.

you can also just right-click on the group in the left-hand panel, no need to select all. (that's wrong, of course).
Displaying 5000 items also shouldn't freeze a reasonably modern computer for longer than a second or two, if they do something else may not be right.

Quieres copiar artículos de tu libraría personal a un grupo? Puedes simplemente arrastrar y soltar artículos o colecciones. Necesitas hacerlo en Zotero aplicación, no en zotero.org

I believe only by exporting the collections one-by-one.
@Dan - is there any reason why there is no right-click --> Export Library option for groups? Seems like that should be there (to the degree that I just assumed it was, see above).

(edit: fix collection/group typo)

@Dan - is there any reason why there is no right-click --> Export Library option for collections

I don't think there's a reason, no. We should add that, and also make Export Library in the Actions menu export the currently selected library (ideally with feedback at the top of the modal dialog about which library is being exported).

Thanks.