I’m not exactly sure if it does NICS specifically. The portal really just checks to see if they have a valid FOID (firearm owner ID, an Illinois special) card, but the foid card is linked to other data bases. I’m not super familiar with all the specifics. Relevant links:

verify.ispfsb.com/Public/Verification.aspx

isp.illinois.gov/Media/CompletePressRelease/679