I had this question since forever (I did not understand it) and I have seen it asked before, so I will post what I learned about it in case it helps someone else.
I am currently a student on baselang, and today I finally met a teacher from a country where estar para is used, Colombia. Estar por is widely used, but estar para is not commonly used in some countries.
Estar para can mean the same as estar por, but there are times when it is not.
Está por llover and Está para llover are both referring to "it is about to rain" or "it is getting ready to rain" which are essentially the same thing
Aquí en la escuela, "estoy para buscar a mi hijo" = Estoy aquí en la escuela para buscar a mi hijo
(which more or less means I am here for the purpose of picking up my son and could be an answer to a question of why are you here?)
This is different from "estoy por buscar a mi hijo" (which is I am about to pick up my son)
otro ejemplo: Estoy para comer means I am here available and ready to eat
Estoy por comer means I am about to eat
Estoy para pasar por el centro – roughly translates to I have to go downtown
Estoy por pasar por el centro – roughly translates to I am about to go downtown
Countries where Estar para is not commonly used
Countries where Estar para is commonly used
I will update the list as I meet more teachers from other countries