"Who, WHAT are you?" *main story spoilers*

[Anonymoose]

Was it explained how Ysayle learned how to summon?

Not explicity, but it has to be the Ascians, doesn't it?

Before Shiva, in Patch 2.3, Lahabrea says that Eorzea has always been "fertile soil for divine seeds" (recall that Lahabrea always refers to primals as "the divine", almost mockingly) and that people will create a new god if they're given the tools and the Ascians "nurture the gifted" (Ysayle). Then, when Ysayle does summon Shiva, Nabriales is looking on and says that Lahabrea will be pleased. And then in 2.5, Nabriales says that he suffered Lahabrea to merge man and god and you undid all his hard work (by causing the deiform to withdraw from the vessel).

It seems too much to be anything else, imho. The only question is how they approached her, and in what form.