Under current regulations, to exit Changi airport, you need to:
- ensure MLE-SIN is a designated VTL flight (SQ437 is)
- apply for VTP (if you're non Singaporean or PR)
- be fully vaccinated and do pre departure test (ART or PCR) at least 2 days prior to departure from MLE (but this requirement is moot, since your final destination is SFO and you'd already have done your PCR test 1 day prior to departure from MLE anyway) or have recovered status within past 7-90 days
- fill in the SG e-arrival card
- set up TraceTogether on your mobile phone
- theoretically, you're supposed to do a self-swab ART and upload your result at the government portal within first 24 hours of exiting the airport (but as your flight is leaving in 22 hours, unsure if this is still required - or if the government cares)

YMMV. These are my best understanding of the current rules, do your own due diligence and assess if it's worthwhile to go through all the hassle.