I heard from a Haredi yeshiva-bocher, and confirmed with another, that most Haredi yeshivos learn only one of the sedorim (or maybe it was one of the masechtos), and that it is the same one across almost all yeshivos (with a few outlier yeshivos known for specializing in another one).
This was quite surprising.
Is it true?
If so, which seder (or masechto) is it?