AnnieBrooke gave you very good information. Just to add - you won't get HIV/AIDS or any sexually-transmitted diseases from fingering.
Sounds like those old myths about masturbation are being recycled about fingering. (It used to be that people would say it causes hairy palms, physical weakness or mental illness. None of that is true; the stories were caused by guilt, lack of information, and someone trying to shame kids into not doing it.)
If those old myths have now popped up about fingering, it's probably for the same reasons -- guilt, lack of information, someone trying to shame kids into not doing it. Whoever you are getting your sexual information from is an unreliable source. Look online for educational websites about human sexuality. For example, here is one from Planned Parenthood: https://www.plannedparenthood.org/learn/sex-and-relationships/masturbation