How to extract code for desired symbols from package into preamble?

I'm trying to follow the instructions here for getting a big 'X' for an iterated Cartesian product: How can I get a big cross to denote a generalized cartesian product?. However, I have no idea where to start.

• Simply load `mathtools` in the place of `amsmath` and use `\bigtimes`. – Bernard Oct 12 '17 at 0:01