This is simply an elided form of
We've improved the visual separation between the header and the body in classes with multiline headers.
Since an indefinite article would not make sense, the definite article can safely be omitted and understood, and people often do so. Each class has exactly one header and one footer, so there is no misunderstanding.
This is an abstract noun meaning "the way that a player interacts with the game" (The collection of rule, goals and how those rules can be changed.) It is not likely to be used countably.
It doesn't mean "an instance of playing a game" Your example could "That was a fun game; I killed all the zombies." or "Also, it could ...
"great advancement in technology" means an overall improvement of technology, technology moving to a higher level.
"great advancements in technology" means several, or possibly many changes in technology, each of which is an imporovement.
In context the meaning is not much different. The singular form is looking at the "level" ...
Are you talking about a single system as a whole, or is the emphasis on multiple systems working together? That's a technical question so I doubt any of us can answer it, but you just need to decide how you're talking about the thing, whether it's singular or plural, and then write it accordingly.
If you are going with the singular version, you need a ...
More is used when making a comparison. (The students are more familiar with PCs than with Apple computers.)
Many refers to a large number of something countable (many students).
Much refers to a large quantity of something uncountable. (The students haven't had much practice at using Apple computers.)
These are the meanings most relevant to what you are ...
Here are my explanations:
Our school only has Apple computers. However, some students are more familiar with PCs.
The above ^ means that some of the students use PCs more often or they like to use PCs more.
Our school only has Apple computers. However, some students are much familiar with PCs.
The above isn't proper grammar IMO, "much familiar" ...