Did I miss something? How to influence the faction of the next population unit on a system?

One time, I was able to change it by moving some population of a planet to another randomly (in the same system). I cannot find a specific rule to be able to chose the next population.

Thx

Maybe this helps:

I was curious about this myself so checked it out. It seems that the chance to increment is a score that goes up every turn and the pop type that has the highest score is the one that will grow. I think I've seen it change sometime by messing around with Starport too but can't reliably reproduce any change in the behaviour. So basically you can't control it unless you transport all your people to other colonies, drop them to Starport where they don't grow or use the luxuries to increase the increment score.

Thx for the screenshot,  it was very helpful. I will check more later in those stats.