When Field Audit Trail isn’t enabled, field history data is retained for up to 18 months, and up to 24 months via the API.

Reference Article:

https://help.salesforce.com/s/articleView?id=sf.tracking_field_history.htm&type=5

If you delete a record in your production data, the delete cascades to the associated history tracking records, but the history copied into the FieldHistoryArchive big object isn’t deleted.

Note:
After you define and deploy a Field Audit Trail policy, production data is migrated from related history lists such as Account History into the FieldHistoryArchive big object.

Reference Article:

https://help.salesforce.com/s/articleView?id=sf.field_audit_trail.htm&type=5