Everything?
His hammering on a "world government" as a solution to all the world's woes and the assumption that it would cure all the ills of human nature by some sort of magic, mimics Socialism without explicitly asserting the obvious.
It's all collected by Heinlein himself, in Expanded Universe.
Yes, I read “Expanded Universe” 30 years ago. The themes didn’t jump out at me as socialist when I was 15, and I haven’t re-read it because short stories can’t hold my interest anymore. I remember more of the characters rebelling against those world governments, as in “Methusaleh’s Children”, “Beyond This Horizon”, “Citizen of the Galaxy”, etc. Those world governments were not portrayed as perfect societies. “Door Into Summer” is pure capitalism, as is “Moon is a Harsh Mistress”. Read the pithy sayings of Lazarus Long in the middle of “Time Enough for Love” and they are pretty much all anti-government.
His FIRST novel, which was rejected back in ‘37, but was found by his biographer and published after his death was what shocked me with its socialist themes having created a utopian future. “For US the Living” I think it was called. It was more a laundry list of ideas than a well plotted novel, some of which were very libertarian, but economically pure socialism. Everybody is on the dole, nobody has any logical incentive to work, and the government just prints however much money it needs to hand out to the people.