Upvote:1
The tours, which were suspended because of Covid, are back as of April 3rd. https://www.changiairport.com/en/airport-guide/facilities-and-services/free-singapore-tour.html has the details. (Note: the Heritage tour doesn't start until June.) All four tours look really good, and match up quite well with things I'd take a visitor to if we only had a few hours.
You need a layover of at least 5.5 hours and it has to line up with a tour you want to take. There don't appear to be restrictions like "your itinerary must be all one ticket". You do need a valid entry visa (since you are going to enter the country, after all) but that is not likely to be difficult. There's a detailed FAQ with links at the Changi page above.