What is the exit and entry procedure for a retirement visa holder traveling to Cambodia?

It depends what you understand is a „retirement visa“

*** if you are still within the 90 days stay permit out of a Non-Imm-O retirement visa, the stay permit will become invalid upon leaving Thailand

*** If you are already on a “1-year Extension of Stay Permit based on Retirement”, which some people call “retirement visa” but which isn’t a visa at all, then you need to buy a single re-entry permit for 1000.- THB on Immigration, to keep your extended stay permit alive when you exit Thailand. So since you mention you already bought a multi re-entry permit for 3800.- Baht, you are fine

No particular procedure as long as you already have your re-entry permits. You check out electronically or manually trough the immigration and get stamped back in on your Extension of Stay (As several calls “retirement visa”).

You might need to update your TM.30 at your immigration, but several immigration don’t require an update as long as you re-enter to the same address. You’ll need to do your first new 90 days report manually as the clock resets each time you reenter the country.

You have a 1 year extension with a multi re entry permit ?

Best to get a E visa for Cambodia. It is a hassle to get a visa on entry, prices can vary depending on where you enter, it is usually a slow process and it takes a full page of your passport. And if at a land crossing you will get bombarded by touts trying to sell you everything from cigarettes to their sister.