Start here:
Population axiology (Hillary Greaves, 2017)
Infinite ethics (Nick Bostrom, 2003)
Further reading:
Population ethics and inaccessible populations (Carl Shulman, 2014)
Moral uncertainty about population axiology (Hilary Greaves and Toby Ord, 2017)
Pareto principles in infinite ethics (Amanda Askell, 2018)
Utilitarianism, decision theory, and eternity (Frank Arntzenius, 2014)
Problems and solutions in infinite ethics (Ben West, 2015)
To me they seem basically the same topic: infinite ethics is the subset of population ethics dealing with infinitely large populations. Do you disagree with this characterisation?
More generally, this reading list isn't strictly separated by topic, but sometimes throws together different (thematically related) topics where the reading list for each topic is too small to warrant a separate page. E.g. that's why I've put global development and mental health on the same page (this may change if I get lots of good suggestions for content on either or both).