Answer 1

Brackets are used to make lists
Braces are used to make dictionary
Parenthesis are used to make tuple

But for indexing in all of those, only brackets are used.

Answer 2

Without further information I would assume that {} indicates a set.

s = {1, 2, 3, 3, 3, 4}
print(s) # {1, 2, 3, 4} (set of unique elements)