how str.length() works, calculates string length()?

Suppose i created a string of size n initially, Question1: Can i change it length() by any method? (since it is just an array of characters, So n must be an upper limit of string) so is there any way of reducing str.length() result?

question2: if so, then how str.length() calculates length, if it calculates by finding '\0' , then if i change any in between character to '\0' , can n be reduced?

I know i asked in very confusing way. But an answer from basics will be helpful! Thank you