How to approach equity at profitable unannounced unicorn startup?

I've been interviewing at a profitable startup and am about to enter the final interview round after the new year. They have around 200 employees, have received several rounds of funding in the past and seem very successful (might be the biggest company in their industry).

I know someone who used to work at that company and alleges the company is a unicorn that hasn't been revealed yet, and that not too long ago, a FAANG attempted to acquire them.

I don't know much about equity and stocks, other than the general advice to treat them like they aren't valued at anything (not counting chickens before hatching and all) and to negotiate for more salary/cash versus equity. This situation, if the particulars above are true, is an anomaly and I'm having trouble finding any info on what approach I should take here.