You’re kind of right, Citizens United did not rule that corporations are people.

That determination was made in the 1880s in Santa Clara County v Southern Pacific Railroad. Kind of, it was in a summary before the ruling, but (like Robert Bork’s essay that said true monopolies don’t exist and even if they did competition would destroy them and so antitrust action should only be taken if a merger will negatively impact consumers where “negatively impact consumers” is constrained to mean “raise prices in the immediate future”) it was treated as having the force of law.

https://uslawexplained.com/santa_clara_county_v_southern_pacific_railroad