Which Process reads and write data from Datafiles?

Reading is Different and Writing is different. Reading is Always done by Oracle server Process and Writing the Data from Buffer Cache to Disks is done by DBWR Process. LGWR writes Transactions into the redo log Buffer and ARCH Process Writes to Archives if the DB is in Archive Log Mode.

In simple Oracle server process read the blocks and DBWR writes into the datafiles but actually both process depends on OS process, means server process and DBWR both are like a mediator.