While downloading and installing applications from official pages on SideQuest is completely legal, installing APKs from unknown sources of official games is piracy and considered illegal. While SideQuest uses protective measures to ensure that no malicious activity is facilitated by the application, you can still sideload APKs from unknown sources. Keep in mind that pirating games is not only unethical but can also land you in trouble with Meta, and you can risk getting your headset bricked.