Ethan H. answered 04/26/20
CASA (Center for Arabic Study Abroad) Fellow and former Arabic teacher
As Alaa said above, in most academic writing, ه is transliterated as h, ح as ḥ and خ as kh.
However, many publications, teachers, and others develop their own standards which may differ. Typically a writer will include an appendix or table outlining their transliteration system. One of the most common standard systems is that used in IJMES for Middle Eastern Studies.
This may vary by field, however. Linguists may use the International Phonetic Alphabet (IPA) and computer scientists may still use others.
In online chatting and social media, Arabs tend to use their own non-standard systems that may vary from dialect to dialect. Some common things you may see are numbers used to represent sounds that the latin script does not represent: "2" used to represent the hamza, "5" used for خ, and "7" used for ح, etc. In Egypt, this is called "franco," because the use of the latin alphabet was introduced during the French occupation. In other countries, this is sometimes called "3arabizi."