how do some sandwiches feel creamy
As far as I know, there are no official categories of sandwiches, although there are considerable regional differences in what is considered a "prototypical" sandwich across the world (and some cultures have multiple common sandwiches). A traditional American sub is very different from a German Käsebrot. So, I can't point you to a type of sandwich and tell you "it will always be creamy". 

There are two things which you can use to make a sandwich creamy. 


Spread butter or cream cheese on the bread before adding the toppings. It will give the sandwich a nice, mild taste, and remove the dry mouthfeel, without making it very moist. 
Use a condiment. Typical sandwich condiments are mustard, mayonnaise and ketchup. They make a sandwich very moist and creamy, and tend to also give it a strong taste. They are normally placed between the toppings, not on the bread, so they don't soak into the bread and make it soggy. 


My suggestion is to try out both variants and decide which you like best.