Math.Abs ​​from string how does it work?

Question

.NET

Good day to all! I came across this example

string str = "d4";
var res = Math.Abs(str[0]);
var res1 = Math.Abs(str[1]);

In this example, Math.Abs ​​takes the value "d" and matches it's encoding (which is 100), right?
Further, if str[1], the value changes to 55.
Can you explain why changing the values ​​in [] causes the results to change?

Answer 1

A string is an array of chars. Accordingly, [i] - will return the character of the string with the number i.

Answer 2

in this syntax, the string is iterated over as an array of characters, [] is interpreted as the index of such an array