Default and temporary tablespaces for installing sample Oracle database schemas? What should I specify as default and temporary tablespaces (users and temp parameters of mksample.sql script) for installing sample Oracle database schemas? 

In my environment I call the script like this:

$ORACLE_HOME/bin/sqlplus SYSTEM/systempw@\"localhost:1521/ORCLPDB1\"
SQL&gt;@mksample systempw syspw hrpw oepw pmpw ixpw shpw bipw users temp /home/oracle/log localhost:1521/ORCLPDB1


Do they exist in a clean Oracle 18 instance? Or should I create them?
You can make them any existing temp and permenant TS you want.  I'll bet TEMP and USER already exists. You can answer your own question of 'to they already exist' by simply querying DBA_TABLESPACES.