What is the theory of a meat marinade?
First, there are two terms for 'soaking a piece of meat in liquid' and they are often used interchangeably and/or in error:


Marinate: A surface treatment of meat in which the meat becomes coated with a flavorful liquid called a marinade (the sauce is the 'marinade', the soaking is 'to marinate', though marinade is often used as the verb).
Brine: A deep penetrating treatment of meat in which meat is submerged in a saline (salt) solution 
(related topics would include rubs and glazes)


Chemically these are two very different actions but colloquially they are often confused. The maximum effect of a marinade can be achieved in a matter of hours and is strictly a 'surface treatment'. A brine is always salt based and takes anywhere from hours to days to reach maximum effect. The first description you provide is actually for a brine, as evidenced by the salt content and duration of the soak. The 'different style' you present is an actual marinade. For a more in depth discussion of this see:

The Secrets And Myths Of Marinades, Brinerades, And How Gashing Can Make Them Work Better

Recipes for either are varied and wide ranging, and I will reemphasize, commonly mislabeled (I'm not trying to be a grammar nazi here, but any discussion of the 'theory' requires a proper distinction) 

As for that 'theory'...there are many opinions on this subject and I'll try to remain agnostic for this answer. Marinades usually breakdown into three components: fat, acid, and spice/seasoning the exact ratio is the subject of great debate but a good starting point is 3:1 fat-to-acid and season 'to taste'. Your fats can be one or more (melted butter and olive oil is a personal fave) as can the acids vinegar, buttermilk, citrus juice, beer, wine... (for marinades alcohol serves as an acid). Beyond that season as you please. Chili powder, red pepper, paprika to get a 'spicy' marinade. Rosemary, dill, garlic, provide rich flavors. So too sugars(white/brown/honey/molasses) will add depth to the flavors and will caramelize and enhance the maillard effect. 

As for pork, in particular, using apple juice (or hard cider) as the acid is nice start, honey (or other sweets) are a good compliment. I personally like clove as a spice for pork tenderloin. 

From here, let your imagination and your palate be your guide. Here are a few useful links to look a little deeper if you please:


Anatomy of a Marinade
How to make any Marinade
A Complete Guide to Marinates Meat


Beyond this...Google is your friend.