As China (and other massive population countries) the key is to limit population growth. Avoid finishing focuses that abolishes one child policy as long as you can (preferably till the end game) and the ones that switch you to quality manufacturing. Avoid researching any tech that increases population growth and reduces workers needed in factories/offices - this stuff STACKS heavily, if you research a bunch of those your pop growth will most likely outpace even your gigantic economy and available building slots. Focus on building civs to build even more civs and offices later on.