When a product is saved either through the UI or using the API, it is also indexed at the point of save/update. The following fields of the product details are indexed and used when returning search results, for each installed and enabled language.
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This is done for the primary product, and then an index is also added for each variant, if they exist.
As you may be able to see from the above list, these are attributes of the products, and not necessarily any specific data or column in the database. That could change on any release.
Anything you don't see listed here is not used as part of the index. Adding something like custom properties would not make sense for most use cases, since those are generally only used for integrations or filtering in the drill down view. In fact including that would most definitely make the search perform more slowly.