Find the option to build a commercial building on the right hand side of the game interface, click on the fourth icon on the right, and then, after clicking on the fourth icon, a row of buildings will appear, so choose the one you need below it and start building.
Then click on the factory you want to build to see how much money you need to spend and what building materials you need, and finally, locate the factory you want to build, and click on the green checkmark on the right after you've chosen it to confirm it.