In the US there is a ground instructor rating. It takes two tests. One is a theory of instruction add the other is slanted toward basic PPL topics. There is also an advanced rating which is more toward CPL topics. To teach in an airliner you generally just need and ATP and type rating because ATPs are authorized to teach ATP subjects. Most airlines hire from within, but a few hire outside instructors and there are a few Part 142 schools - Flight Safety is one that give type ratings.