@TheAviationGallery nailed it.
Shortly, they are two airports.
A bit longer.
WSSS is the older one of both. It is a civil airport that hosts a small military part in the west, Changi Airbase at this time.
With traffic increase for bot civil and military the new changi airbase was build east of the airport, that’s WSAC, called Changi East Airbase and Changi Airbase became Changi West.
A few years ago they have closed Changi east to renew and extend the runway. One of the reasons for this where plans for renovation of runways and expansion of Changi International WSSS.
With the Airbase Runway WSAC reopen it had already all connections for a taxiway west of it.
This taxiway is now finished and connected to WSSS. That’s called the northern loop. The taxiway west of the WSAC runway is already part of WSSS.
With the airbase WSAC connected to the civil airport WSSS it is now posible to close one of WSSS runways for renovation without serious traffic cuts. In theory.
Organisational WSSS and WSAC have their own tower and ground control. Approach is shared.
When Approch sends you to the eastern runway, the one belonging to WSAC, you will contact WSAC tower for landing, and if you are civil traffic you will be send west of the runway. When you left the runway you contact WSSS ground for taxi.
There are some other airports in the world where those situations ocure, RJCC and RJCJ where one example. There are others like HESX and HECW in Cairo Egypt.
In case of Chitose they where once made as a single airport for IF. But with navigation data coming from a external source we had to split them to have the correct ICAO for the different runways. Without that you wouldn’t been able to use the SID and STARs as well as ISPs of one of the parts.