Extract string at key function

#1

why did we pass string str ?? instead of str can we use any variable ? if not then what is the use of str ?
also please explain typecasting that was used here ? its syntax and working

#2

Please mention the timestamp in the video so that i can check what you are referring to.

#3

8:43 is where he inputs str and 9:46 he talkes about typecasting

#4

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