Question

In a dynasty how do people decide who will become the next ruler

Answer 1

That varies per dynasty and circumstances.

Usually the the ruling dynast (king, emperor, etc.) decides that. This is also based on customs. Most often, the eldest legal son. But not always. Sometimes there are no legal sons, only daughters. Or the obviously better choice is a lesser son, while the most likely successor can't tie his shoes unassisted.

If the dynast is not very strong, his prime minister or crown council can decide. Often, the mother of the dynast or one of the heirs proves to be the real kingmaker.

Answer 2

In a dynasty people do not decide who will become the ruler . The king's son mostly becomes the king after king's death