Reading from the Oracle RDMS is done by the users server process
Once a query is submitted, the users server process first determines if data is in the buffer cache of the SGA (System Global Area) memory area.
If the data is not already in the buffer cache, Oracle reads the data from the data files on the disk and writes it to the SGA.
The data is returned to the user or manipulated in memory from there, not from disk.
This means that commonly used data can be accessed form the Buffer Cache which is in RAM and very fast and not form the disk which is slow.
This is a very scalable architecture.