If you're asking how to get everything after the '@' symbol, you can use knowledge of UTF-8 (assuming your byte array is UTF-8) to just say `FindIndexOfByte(utf8Buffer, (byte)'@') + 1`. But treat everything else as an opaque blob. The URL portion of that email address, as far as you should be concerned, it just the
bytes after the '@' symbol. If you want to verify if it's a valid URL, you should use a URL library, not some hack that assumes a TLD is 3+ characters. However, even that naive method of a byte search is wrong. According to RFC 2822, '@' symbol is actually allowed on the left hand side if escaped. You should be using an email parsing library that implements the spec properly.
Naming the smallest unit of information in C a 'char' was a terrible mistake made decades ago. A character has never been a single byte in many countries. Shift-JIS, for example, can be one or two bytes, and it's two and a half decades old. Even in UTF-16 (used by Java and C#), a 'char' isn't actually a character, but a codepoint. One that could even be an unpaired surrogate.
Really, you should never be indexing into the backing byte/char array unless you know what you're doing. If you think you need to, you should be asking yourself, "what is it I really need to do?" Because you, as a programmer dealing with internationalization, must not assume English/Western ideas about language. For example, in abugidas[0], a single word can be a whole sentence. For a more common language, Japanese typically doesn't use spaces because they're not needed when writing kanji. But that's not foolproof because hiragana and katakana aren't for words, but sounds. How do you do a word count in those languages? You don't.
[0]: https://en.wikipedia.org/wiki/Abugida