Always, when I see someone doing this projects I wonder how easily is to do legal stuff with projects that don't offer you a high revenue. How do you pay taxes

Always, when I see someone doing this projects I wonder how easily is to do legal stuff with projects that don't offer you a high revenue. How do you pay taxes for a international business? Do you have to pay in every country that you have s costumer? Also, how do you make sure everything is legal in every country that it's accessible?