Because Afrikaans normally writes compound nouns as a single word.
So where English often writes:
Afrikaans typically writes:
This is very common in Afrikaans. English speakers often want to separate these words, but Afrikaans usually joins them.
The -s- is a linking letter inside the compound.
So the structure is roughly:
That -s- does not usually need to be translated by itself. It is just part of how some compounds are formed in Afrikaans (and Dutch as well).
The safest approach is to learn the full compound as a vocabulary item.
Sometimes yes, depending on what you want to emphasize.
So:
Both can be natural, but is is more general.
The most likely plural forms are:
Notice that die stays the same, and is also stays the same.