It is not surprising that there is no formal on-boarding process for the company with just 20 people. While it is good to learn from others, you must keep in mind that this process will be very different in its core for a small company (in terms of number of people) compared to a large company.
For a small company (in terms of number of people) like yours, it is more like a family and everyone usually has personal relations with everybody else. It is more effective to keep that way. There may be any need to have a formal elaborate process of on-boarding except explaining some HR policies and procedures, as long as company culture is of openness and collaboration. At this size, on-boarding is best done on the job by the whole team. The new person should feel welcomed by everyone and should feel part of the family.
May I ask, why do you ask this question? Do you think your current informal on-boarding is not working?