Marx is saying: worker produces X value and is payed Y, and since Y < X, bosses are evil, capitalism is evil, blah blah blah
Well, to begin with, the situation where X < Y is fatal to a company. So it's never going to happen (theoretically)
So X - Y, is what? Pure profit in bosses pocket? Not really...
It's the cost of everything needed for the company and for the worker to do their job
It's the job security (even a tiny one like in some states in the US)
Yes, whatever is left is the profit, and a small profit times the number of employees is the profit of the company, and if someone thinks it's unfair they're welcome to start their own company and compete (which is why cartels, oligopolies, monopolies are so damaging)