When a child is adopted, this ends all of the birth parents' rights and responsibilities. A birth parent cannot just reappear one day and try to reclaim the child.
Guardianship does not sever the rights and responsibilities of the birth parents, but means that a caregiver is responsible for the care and custody of the child.
Legal custody is different than guardianship in that it may be easier to receive government services from the state if you are a guardian rather than a legal custodian.