Swallowing your fingernails is not harmful to you but fingernails are not digestible by the body so when you swallow fingernails the fingernails will just sit in your stomach until it's ready to be passed through your intestines and out through your poop.
Biting your fingernails off though can wear down your teeth enamel and cause tooth damage as well so it's best to not bite or swallow your fingernails.
It is bad to chew on your finger nails because chewing on your fingernails can damage your teeth if you do it enough.
Also chewing on your fingernails can damage the skin around your fingernails and cause bleeding sometimes which can also lead to infection.
Your fingernails can contain bacteria and germs that you cannot see and when your bite your fingernails you can introduce those germs and bacteria into your body.
That can then make you sick and so if you bite your fingernails you should try to find something else to chew on such as a straw that is clean or something else.
Chewing on fingernails is a common thing people do out of boredom or when they are nervous or suffering from anxiety.
I used to bite my own fingernails in Kindergarten because I was so nervous and also did so at home.