In UNIX they are stored in the location $ORACLE_HOME/dbs and ORACLE_HOME/database for Windows directory.
Oracle by default starts with SPFILE located in $ORACLE_HOME/dbs.
If you want to start the database with particular file, we can append it at the startup command as
SQL > startup PFILE = ‘full path of parameter file ‘;
You can create PFILE from SPFILE'
SQL>create PFILE from SPFILE;
update parameter in PFILE then run the following command
SQL>Create SFILE from PFILE;