Instead, Jewel is located on the landside (the part of an airport before passport control). This means, in order to see it, you need to clear customs and passport control. Here's a map of Changi Airport, including all four terminals and Jewel, to help you get a better sense of where it is, exactly.

Jewel Changi Airport is located before security, in the public "landside" area of the airport. It is a massive multi-dimensional lifestyle destination designed to be accessible to everyone—travelers, locals, and transit passengers alike. Because it is outside the secure area, you do not need a boarding pass to enter the main forest valley or see the HSBC Rain Vortex (the world's tallest indoor waterfall). However, this means that if you are a transit passenger arriving from another international flight, you must clear Immigration and Customs to visit Jewel, which requires a valid entry visa or transit visa for Singapore. Once you have finished exploring Jewel, you must clear security again to reach your departure gate. For those who want to see Jewel without clearing immigration, the automated Skytrain that connects Terminals 2 and 3 passes directly through the heart of Jewel, offering a brief but spectacular view of the waterfall from the secure "airside" zone, though you cannot get off the train inside the building.