What is the difference between a built$-$in function and a function created by a programmer?

Multiple select question.

The code for a built$-$in function is included with the Python software and is hidden from programmers using that software.

User$-$created functions are required of all programs, regardless of the length of the program.

Built$-$in functions perform specific tasks. Programmers can create their own functions to do other tasks, as needed.

Built$-$in functions cannot be used when a programmer creates a similar function.