Just like how capitalism is a spectrum (in that, y'know, the US does not have purely free market capitalism but is still widely considered a capitalist state), socialism is also a spectrum. Marx and Engel's writings do touch on this, but it was expanded on by the works of Lenin and Deng Xiaoping (among others). If you're actually willing to learn, I'd recommend asking on hexbear.net because they're much more familiar with the literature than me.