I don't know about FnL, that is before my time :-) 

OC, 2SXC and LV all use json, AFAIK. 

2SXC and LV can retrieve data from the DB as a source. 
If you want to use OpenContent and also query the database, you might want to take a look at https://github.com/EasyDNNsolutions/DNNTokens

The tokens from this extension can also be used in OC. 

Any generic data that is based on template/generic fields is going to be more complex than any standard database project, that is the price you pay for dynamic data.
Like Tycho says, some use json, some use a standard table arranged with primary ID and parentID (I think DNNsharp use that), OpenStore and RocketCDS use XML.
I don't think any use a standard format defined by DNN. But I may be proven wrong.

To overcome the data extraction problem I created a set of views for OpenStore, that looked at the XML in an easy to understand way. However, I never heard of anyone using it apart from me, so I did not bother doing that for RockteCDS. The difference between json and XML is that while json is popular, jpath is not as good as xpath, but both can be easily converted to whichever you prefer.

Creating a set of views for each system may be an option for you, but with difficult structures it may be a hard and long process.

Most modules use a unique DB schema, but usually in the same database as DNN itself. Not many extensions made use of the underlying ContentItems data structure, although DNN makes use of it a lot. Developers find it easier and faster to just define their own tables.

Thank you all! That's what I suspected. I'm a novice developer, but the more I use structured content, the more I feel it's wonderful for somethings, but for others, like Will says, it's easier and faster to create your own tables. 

Always appreciate the knowledge and participation here. Thanks again. 

John