Why are bit masks called “masks” and what purpose do they serve

If you're using a two's complement signed integer, then the first bit indicates the sign, and the remaining bits count down from -1, with -1 represented as all "1" bits, so, counting down, you have:

 7: 0111
 6: 0110
 5: 0101
 4: 0100
 3: 0011
 2: 0010
 1: 0001
 0: 0000
-1: 1111
-2: 1110
-3: 1101
-4: 1100
-5: 1011
-6: 1010
-7: 1001
-8: 1000

The generic definition of "data structure" is anything that can hold your data in a structured way, so yes this would include composite types and primitive types in addition to abstract data types. For example, a string is a data structure as it can hold a sequence of characters in a structured way.

However, the term also has another meaning to programmers.

Since the term "data structures" is so broad, developers usually use a more specific term to identify what they are talking about, such as class or data object or primitive type, and the specific term used for most complex or abstract data types is "data structure"

This is why you hear "data structure" most frequently being used for abstract data types like Arrays, Lists, Trees and Hashtables, and not for things like primitive data types