An internist is a doctor that focuses primarily on adults and you do not need a referral to see an internist.
However your current doctor can give you a referral to see an internist if you want to or need to see an internist.
An internist doctor usually focuses primarily on working with and treating adults while a doctor that is not an internist may focus on both adults and kids and treat them both.
The difference between a doctor and an internist is that the internist works with adults and focuses exclusively on adult medicine.
A internist is a doctor but usually only works on adults while the doctor or family doctor works on both adults and teens and children.
So that's really the only difference as an adult will usually see an internist especially if they are alone and have no other family.
Sometimes a family may see the same doctor as they want to have a family doctor.
Then some kids go to a pediatrician which is a doctor that focuses on kids and kids medicine.