Political Science vs Public Policy vs IR

I’m a junior who’s looking to apply at a few schools (umich, Princeton, Harvard, Georgetown) but is confused on the difference between poli sci, public policy, and ir. My dream job has always been to be a diplomat or work for an international organization, so IR always seemed like the ideal route, but now I’m not sure, as I could see myself being involved in domestic politics just as easily. Also, while umich is probably my number 1 school, they only offer an international studies major, which doesn’t seem to hold a candle to the poli sci major which is offered at umich and is very highly ranked. To add, public policy seems interesting but I’m not sure if it’s too narrow of a focus. Would political science incorporate elements of the other two inside of it, such as current foreign policy and domestic issues, or is it just a history and study of politics?? My stats are 4.0 gpa unweighted, 4.25 gpa weighted, 34 act. Thank you any help would be appreciated