It really depends on what type of fields you’re using.
Basic fields (anything listed under “Basic” for field type) in ACF store text information, beyond that it gets complicated. To be honest, I don’t even know what’s in the database for all field types.
The “meta_key” value to use can get complicated if you’re using repeater or flex fields.
It would be a very long post to go into every type of field. If you can give more information about exactly what kind of fields you’re dealing with that would make it easier. In most cases the way I’ve figured it out was to open up phpMyAdmin and dig around to figure it out.
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