SIN Transfer
 

Planning to fly NRT-SIN (Delta), land at SIN at 11.30 PM. Plan to buy a separate ticket for SIN-MAA next day for flight that leaves at 5AM. Should I have to clear Immigration or can i stay inside the airport? I guess I can walk from T2 to T3 or others without the need to get out of the terminal?

change of terminals
 

Changing terminals in SIN does require clearing customs & immigration. I did it recently transiting SIN on UA (SFO-SIN) to Air Asia (SIN-KUL). After immigration, I took a inter-terminal bus from T2 to T3. It was pretty quick.

yes you can walk from one terminal "airside" to another. Bear in mind that the terminals are in a U shape. with Terminal 1 in the middle and Terminals 2 and 3 on either side, so depending on where you are walking to/from it might take you a long time to walk.

This is not correct. T1/T2/T3 are connected airside and you can walk between every gate. SIN T4 is in a separate building, and there is an airside bus.

The question is whether your onward airline (from the clues you provided, perhaps it is IndiGo?) offers check in service at a SIN Transfer Desk. If so, you can stay airside (never having entered Singapore). But if not, plan to explore the airside airport facilities for a few hours, and then enter Singapore immigration at perhaps 2-3 am, and proceed to your landside airline checkin counter to check in, and then back airside for your flight.

Flight Transfer

Thank you. Yes I am planning to buy the Indigo Ticket