Tech explanation: I am not keeping a tight lip on the technology calculation really, my explanation has just been scattered all over the place. But at least what matters is: Religion, Average Technology of Realm, Nation(Ottoman and Aztec get some extra love), and if you hold key provinces. To give an example: If a Muslim conquered Paris, he will count as ottoman tech group. If he at the same time is very high tech(average tech level of realm is 5 or above) he will get the western tech group. The same counts for Tengri, they start as nomad, reform will give them Chinese, high tech will give them eastern and if they hold key provinces they will become western. All the other pagans have it easier. They only have to hold key province OR have high tech in order to get western.
Also important to note, this is average tech level of your
Realm and not your capital. Which makes it a bit more difficult and which is why if you want it, you have to actually play the game and not just start up a random bookmark