finding a replacement habit isn’t always so easy. If you are biting your nails because of the chewing or the oral component, sometimes chewing gum will help. I’m sure in some circles it’s worse than nail biting but you get the point. If it’s about the peeling sensation, some people have put on layers of nailpolish that they can peel off when they get that urge. (Clear polish doesn’t always show the damage). We used to pour glue all over our hands and let it dry so we could peel it off, that was a great replacement but not real convenient.
That was what I meant by finding out what it “does for you”. One thing that worked for someone was to bite down on their fingertips (not hard enough to hurt, just to really feel it) every time they put their fingers to their mouths. It was still the same kind of “stimuli” but no damage done.