When you visit a website online or use an app, which country's laws should someone abide by?what will happen if a website is selling something illegal according to their country's laws online but it is legal to buy in your country?
If you are physically located in country X you are subject to country X's law, whether you are online or offline. If you build a website offering services to people in country X and it has to be legal according to country X, no matter where you are located or the website is hosted.
In theory the laws are very straight forward but in practice it can be very hard to enforce them.
Edit: Some examples. Suppose selling marihuana via a webstore is legal in the Netherlands but illegal in Poland. If a Dutch webstore mails a package to Poland, the webstore owner is breaking the law (and the customer in Poland is as well). This kind of situation is somewhat enforceable because one can track the package back to the sender. Suppose watching pornography is legal in Germany and illegal in Saudi Arabia. If someone in Saudi Arabia goes to a pornography website hosted in Germany that would be illegal but that is almost impossible to enforce without massive internet censoring/filtering.