Downloading and installing iTube is not illegal unless your country has expressly prohibited its use. However, this app does violate both the terms and conditions of YouTube, which prevents it from being available on the Google Play Store. You can still get iTube from other sources, such as Malavida or the Galaxy Store, the Samsung app store.
On the other hand, it is the responsibility of the user to use this tool lawfully or unlawfully. For example, it can be used to play content, without storing it in the memory of the device. In this particular case, the function would be very similar to that carried out with the official client. On the other hand, it can also be used to download music or other royalty-free resources. Finally, it is a good tool for retrieving content that you have created yourself.
Finally, you must review the laws of your country regarding this matter. It is possible that some countries are more permissive and others more restrictive. Therefore, before using iTube, make sure that there is no legislation about it or similar applications. If you have already made these checks, you can get iTube quickly and safely with the link on this page.