Understandable. Socialists need a glossary, lol. We use a bunch of terms basically nobody else does.

Communism, in terms of a state of being described by Marx, isn't utopia. It's just a predicted transformation of how humans relate to each other, and thr economic system under which they live, created through a basic liberation: what if the people who work to make all the stuff got the reins of power? And while the logic is more complex, the basic idea is that they would make their own lives easier and they would prevent other classes from taking over, and because of the nature of how stuff is made, that would result in the abolition of economic classes and high levels of production that sustain rich lives with less work over time.

Marxist thought attempts to reject idealistic thinking and instead ground its ideas in what is truly possible relative to how power and economic relations really work. It does not require any assumptions that humans are purely altruistic or anything like that.

With that said, humans have far more capacity for mutual cooperation and sharing and equitable justice than is commonly believed. Under capitalism, there is a cult of greed that tries to depict the extractive and violent relationship its own ruling class has with the others as a natural and even beneficial thing, and this cult of greed is very popular for a variety of reasons, the most important of which is that it keeps people from directing their frustrations at the party responsible for the aforementioned extraction and violence (the ruling class). This cult of greed is conflated with "human nature" despite the fact that both current and past societies exhibit all kinds of variation in how people relate to one another, and the most common forms for the longest periods of time were built on mutual giving and soft debts that were often communally written off.

Buy communism doesn't even really depend on societies becoming particularly altruistic like a light switch gets flipped or anything like that.

Finally, I should mention that communism is framed more as a long-term eventuality, and one that requires work and struggle to achieve. No communists expect to see it in their lifetimes. Instead, we expect that we can instead achieve socialist revolutions, which put the working class on top, a necessary precondition and also a massive intrinsic good in itself, as you can see in how peoples' lives are improved in countries run by socialists.