A cell is like a factory because of all the working parts, or organelles inside of it. In cells, they produce, and export different things, and also use up energy and create it.
There are different organelles in a factory, like the golgi apparatus, that pack up things to ready them to be shipped, like in a factory.
There is a mitochondria which powers the whole entire cell, or factory in this analogy.
There's ribosomes which create proteins, as well as vacuoles which can transport them to other places.
Hope this helped :)