Normally, a person who meets a state’s eligibility requirements for the sealing of juvenile court records has to file a petition with the court to actually get them sealed. The person with the record often has to file such a petition (accompanied by payment of a fee) with the juvenile court clerk in the county where the case took place.
Typically, the petition formally asks the court to issue a written order sealing the record. However, some states do have limited automatic sealing provisions. (See, for example, Cal. Welf. & Inst. Code § 826(a), discussing in part automatic record destruction.)