How does Go handle strings, runes, and bytes? Why is len(s) not the character count?

s := "Hello, 世界"  // UTF-8 string containing ASCII + Chinese chars

// len() counts BYTES
fmt.Println(len(s))                       // 13
// 7 ASCII (1 byte each) + 2 Chinese (3 bytes each) = 13

// Count RUNES (Unicode characters)
fmt.Println(utf8.RuneCountInString(s))    // 9
fmt.Println(len([]rune(s)))               // 9 (same)

// Byte-by-byte iteration — WRONG for multi-byte chars
for i := 0; i < len(s); i++ {
    fmt.Printf("%d:%x ", i, s[i])  // raw byte values
}

// Rune-by-rune iteration — CORRECT for Unicode
// range decodes UTF-8 runes automatically
for i, r := range s {  // i = byte offset, r = rune (Unicode code point)
    fmt.Printf("%d:%c ", i, r)
}

// Conversions
b := []byte(s)      // string → []byte (copies)
r := []rune(s)      // string → []rune (copies)
s2 := string(b)     // []byte → string
s3 := string(r)     // []rune → string

// Efficient string building (avoid + in loops)
var sb strings.Builder
for _, word := range []string{"Go", " ", "rocks"} {
    sb.WriteString(word)
}
fmt.Println(sb.String())  // Go rocks