They're discovering their hands and also chewing on fingers while it could be a sign of teething it's also them getting ready for solids/purees by strengthening their jaws/tongues/whatever else and desensitizing their gag reflexes. My son loves to chew on his hand then spread all the spit across his face and at this point I've just learned to let him be covered in drool
my son does the same thing he’s 4 months it’s normal. they are discovering their hands!! make sure you don’t put him to sleep with a bib on though! i didn’t know that at first