The following diagram (source: https://www.embarcadero.com/products/rad-studio/firedac) summarizes the architecture of the FireDAC library and you can clearly see how the library is layered, that is, on the top, there are FMX and VCL applications, using the library to implement data access and management:
As you can see, at the bottom, there are the DBMS client libraries (provided by each vendor and part of the DBMS software itself) and in the middle, there is FireDAC, itself layered from drivers (physical or bridging) to interface with the DBMS client libraries up to the higher-level components (non-visible and visible).
The most important and most used components, such as those implementing a database connection, transaction handling, SQL execution, and result-set fetching, are all situated in the Non Visible Components (Comp) layer and take advantage of capabilities and functionalities implemented in the underlying layers...