quote produces a piece of code represented as AST. The num in the AST is not linked in any way to the num = 1 declared before it:

iex(1)> num = 1
iex(2)> ast = quote do: 1 + num
{:+, [context: Elixir, import: Kernel], [1, {:num, [], Elixir}]}
iex(3)> ast2 = quote do
...(3)> num = 2
...(3)> unquote(ast)
...(3)> end
iex(4)> {result, _binding} = Code.eval_quoted(ast2)
iex(5)> result
3

When building the AST you can use values from variables by using unquote:

iex(1)> num = 1
iex(2)> ast = quote do: 1 + unquote(num)
{:+, [context: Elixir, import: Kernel], [1, 1]}

Similar how you can think of quote/unquote as here:

iex> ast = "1 + num"
iex> ast = "1 + #{num}"

Assignment is a special form and indeed it’s a macro.