These are delivered to the streaming service from the production company supplying the content. Depending on the production company they can have various sources. Some are made from the shooting script, some are transcribed either manually or via ai. Some go through a full quality check before being delivered, some have no QC at all. It all depends on the production company. Theoretically most streaming services say they quality check the delivered subtitles but this is usually done automatically with very occasional manual checks. For subtitles not in the original language these are usually contracted to various subtitle translation companies, usually by the streaming service, but sometimes by the production company depending on the contract. These can vary in quality and price immensely.