How this code can calculate bytes of a string ?

Question

How the line 3 of this code can calculate bytes of given string ? What is the logic and how ?
https://code.sololearn.com/c8tHix009IA0/#py

Answer 1

Python strings are null terminated so it writes each one counting them until the '\0' is found

Answer 2

One letter is one byte (including spaces). "Hello world!" is 11 letters plus one space