His military ranks are also fairly automatic but reflect the fact that he does carry out ceremonial duties for all the forces from time to time.
And the letters after his name do not relate to his medals - although some of them do relate to his stars, which as I said, he has received because of his role as a Prince. But let us look at his medals specifically.
The picture you shared makes it a little difficult to identify them all, but I will do my best. I have found a larger version of it with watermarks here:
The first medal he is wearing is the Queens Service Medal, which is a New Zealand medal which is awarded by New Zealand for public service to the Crown. The Prince has been serving the Crown his entire life. It's not explicitly a military medal.
The second medal he is wearing is the Queen Elizabeth II Coronation Medal. This is a commemorative medal awarded to those who participated in the events around the Queen's Coronation in 1953, as well as selected people from Commonwealth defence forces at the time. He received it for participation in the events surrounding the coronation - which he did. Well over a hundred thousand such medals were awarded. Again, not explicitly a military medal.
The next three medals are similar to that one - they are the Silver, Golden, and Diamond Jubilee medals, given to those who participated in the events surrounding those jubilees. Again, not explicitly military medals.
The next is the Canadian Forces Decoration with 2 clasps. That is given for 32 years service of any sort in the Canadian Forces (12 years for the medal - a clasp every ten years after that). The Prince is honourary Colonel of a number of Canadian regiments and that is qualifying service.
Then we have the New Zealand 1990 Commemorative Medal, which the New Zealand government awarded to people they deemed to have provided service to New Zealand over a long period of time, which the Prince has done in various ceremonial roles.
And finally the New Zealand Armed Forces Medal given for fifteen years service with the armed forces of New Zealand in any capacity, including as Honourary Colonel of some New Zealand Regiments.
You'll notice he doesn't have any Australian long service medals - that is because the rules Australia applies for such medals specifically state it's not just any service that qualifies but specifically service the permanent or reserve forces - service as an Honourary Colonel doesn't apply. But that just serves to illustrate how he does have qualifying service for the others - the letters patent that define the qualifications for those medals includes the way he has served.
None of these are prestigious medals - commemorative medals, especially, are more or less a form of special souvenir. The practice of awarding them may seem a little odd, but it's a longstanding tradition.