The library cache is where Oracle Database stores executable SQL & PL/SQL code; i.e. the actual instructions the server needs to run the statement.
So
since it was brought into the library cache => from the time it was parsed and these instructions were created.
V$SQLAREA returns the FIRST_LOAD_TIME & LAST_LOAD_TIME to show when it was first/last put in the library cache.
You might find these architecture diagrams handy for seeing how the database fits together (go to Database Server -> Database Instance -> System Global Area -> Shared Pool to see the library cache).
https://docs.oracle.com/en/database/oracle/oracle-database/21/dbiad/index.html