From each according to his ability, to each according to his needs.

First of all, I will say I’m definitely no expert on communism, but it’s definitely not true that “I don’t know a thing about it”.

As I mentioned, there are quite a few views on what communism is. Communism precedes Marx and Engels and there is even a small book from Engels which discusses previous views of communism (called utopian) to their view of communism (called scientific). The phrase you mentioned precedes Marx and Engels’s work and they study how that phrase could become true. In their work, Marx and Engels do mention scientific communism cannot be exclusively theoretical (which they call praxis), for the risk of being utopian. So according to past and current experiences (USSR, China, Laos, Vietnam, Cuba, North Corea), there are quite a few developments and different views on communism. They don’t all agree on everything, but they do agree on “not having privately owned means of production”. On the Stalin era of USSR, it was considered something similar to the phrase you mentioned, but it was somewhat inneficient. People need incentives for their work and discoveries and it was not based exclusively on needs, as that phrase implies. The reality was complex, btw. This is really a generalistic view and don’t expect it to be flawless.