Understand that there are two types of ETF in singapore, SPDR STI ETF and Nikko AM STI ETF. Then what are STI index (https://sg.finance.yahoo.com/quote/%5ESTI?p=^STI) . Are they different?

You can see the tracking error since inception of the Nikko one here somehow:

STI 500 versus the 2 STI 500 ETFs

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The STI index is basically the top 30 companies listed on the SGX based on market capitalisation. The two ETFs that track the STI basically offer a simple way to invest in these companies via a fund, rather than going out there and purchasing shares in all 30 companies separately. How the two differ are via their expense ratio and how closely they track the STI itself