Opening an iModel in a SQLite browser shows that it's just a normal SQLite file. The table structure is what makes a SQLite file an iModel. There are tables with a number of prefixes, the two most important prefixes are bis_ and ec_.
The bis_ tables hold all elements, aspects, models and relationships stored in the iModel and can be queried using the BIS schemas.
Some of the columns in the bis_ tables will make sense on their own but the columns very quickly start having names like js42 and are of type BLOB. These columns are used to store a different property value depending on the class of each row. This column reuse is what allows an iModel to efficiently store Elements of an arbitrary number of classes in a fixed set of tables. It also allow efficient polymorphic queries and fast joins between related Elements.
The ec_ tables hold the class information that map from the bis_ tables to the classes defined in the BIS schemas. The information in these tables is used to power ECSQL and is queryable using the ECDbMeta schema.
