Ortloh has only a few small groups of Yuan-ti and one large group living in the Jungle Pyramids. In other words, the Yuan-ti are really rare in Ortloh.
The Yuan-ti arrived in Ortloh several millennia ago and none of them really know how that happened. A few scholars who have specifically worked on Yuan-ti origins have two theories as follow.
It looks like a pair of Malisons flew here on an Illithid ship in the form of large snakes. It looks like the Illithids did not notice them in the ship. Either that, or they saw large snakes and didn't mind such hoppers.
The second theory, which is less plausible, is that a Yuan-ti god sent a small group of primordial Yuan-ti to the world of Ortloh. This group would have gained a lot of power quickly and built the Jungle Pyramids.
The smaller groups are composed of a few Madisons and Pureblood Yuan-ti.
Two of these groups have an abomination.
TODO: mark their locations on the map.
The Jungle Pyramids is a strong hold of Yuan-ti in Ortloh. The area has five pyramids organized as follow:
O O
O
O O
TODO: draw an actual map.
The center pyramid is much taller. Since the pyramids were built, the Jungle has taken over the area. A long time ago, the smaller pyramids were visible from the center pyramid and the tall pyramid could be seen from each smaller pyramid. Now, there is too much jungle in between.
The Yuan-ti are able to visit four of the pyramids through tunnels. As a result, the fifth pyramid is not used as much (top-right on the picture above, which would be East on a map if I'm correct). It is primarily used to meet with people that are not Yuan-ti. You only find Slaves, Purebloods, and Malisons at that location.
The other pyramids host Slaves, Broodguards, Purebloods, Malisons and Abominations. Many areas also have Bone Naga as guards. Near the top of the other three small pyramids are eggs being attended by Broodguards.
The central pyramid is where Zozz lives. Zozz is a powerful abomination who is persuaded he has the perfect way to become a demi-god (he really will just become an Yuan-ti Anathema, see the Jungle Pyramids adventure for details about this).
The city of <TODO:name> has a large cult that venerates snakes. This cult expands East toward the Jungle Pyramids.
The cult allows for the Yuan-ti to get enough additional humans to become Purebloods and grow the number of Yuan-ti in the jungle.
For the most, the food supply and Broodguards come from tribes of people living in the forest and controlled in various ways by the Yuan-ti. Although the cult of the Double Headed Snake helps with organizing these people, they are not really linked to them.
A very few humans, after years in the Double Header Snake Cult, can become Yuanti themselves. These humans are transformed in Pureblood Yuanti, but they get the title of Youngblood, which is derogative for "you were human before".
Note: When a player character plays a Yuanti, they are not likely a Youngblood.