Can anyone knows what are this three in script. How to create or find it? BEFORE LOAD FUNCTION BEFORE SUBMIT FUNCTION AFTER SUBMIT FUNCTION

I think the best description is here: User Event Script Execution https://tstdrv1234795.app.netsuite.com/app/help/helpcenter.nl?fid=section_N2962434.html This is in the help and explains how each function operates in the user event context.

Those are the entry points for user event scripts. They are named functions, and key/value pairs for the return of your define callback for the user event script type.

I see some existing implementation in my netsuite. How can I view them and test it?

Your script will run on a record type. Before load will run before you load a page for a record of that type. The others will run when you hit save.