I do not think that the free spins have some great probability for winning than regular rotations.
I believe free spins are just like regular spins, you just get them for free. When I play slots, I usually go for the bonus buy, which gives some free spins, and honestly, it’s no different from betting a fixed amount and hitting auto-spin. If you’re lucky, you win, simple as that.
Games like this aren’t something to overcomplicate as it’s pure luck. Your only real chance to win is if luck’s on your side, so there’s no need to overthink slot probabilities or whatever.
I'm not sure if the winning probability is the same of regular spins, because most gamblers I see rolling those free spins end not winning any substantial prize. I have the impression free spins are configured to give only small rewards, besides their winning chance being inferior. After all, free spins are just an encouragement for gamblers to continue playing and depositing more money, since it's pretty addictive to receive bonuses like free spins from times to times.