tech for light tanks lets you built medium tanks.

add sloped armor and learn heavy tanks and you can build heavy tanks.

Guessing that "light" should be "medium" or did i miss something?

"Light Tanks" covers a number of tanks for each nation, culminating in what are now known as "medium tanks" like the PzIV. They were based off the same methods as creating light tanks.

Heavy tanks were made completely differently.

Ah process not product.