Every character has a unique number called a code point, usually written in hex.
Unicode assigns each character a unique number, like U+0041 (A). The value after U+ is hexadecimal.
Enter a value in this tool and it also shows the character for that code point.
ASCII characters (0-127) share the same value as their Unicode code points. For example, 'A' is 65 (0x41).
UTF-8 stores a code point in 1 to 4 bytes. The ASCII range uses 1 byte; other characters use several bytes.