Verified answer

Just a theory of mine, but it can explain a lot of what's happening:

- Reiner and Bertholdt were already born as titan shifters (unlike Ymir, who got her shifters power after swalling one possible villager of this titan shifter village, who was friends with reiner and bert), and because they lived in a village of shifters, they learned since early ages how to better handle this transformation. Ah, and they were created to fulfill one special mission, which was to blend in with the people inside the walls (in order to do that, they had to destroy the walls, allow a bunch of titans inside so they could transform back without anyone noticing) so that they could either retrieve the coordinate (if by any chance it was there) or kill everyone (since every human being can be transformed into a titan) to keep more titans from attacking their village. But they found eren, who has a power that once belonged to their village but eren's dad stole it and gave it to eren, and since he was wanted he had no other option but to go to inside the walls.

But, who is the ape titan? Why does he exist? Why does he want to wipe out mankind? These are questions I can't answer.

Firstly, they have been titans, or titanformers or whatever, for a longer time, and secondly I suspect they have received training from people who know what titans really are.

And stronger than Annie... not so sure. They each have their specialty. Annie/Diva is fast, smart and agile; Reiner/Ironclad is armored and strong; and Berthold/Colossus is of course gigantic and more powerful than any other known titan.