Which Unicode symbol that normally looks like two lines crossed together properly represents multiplication and what do the other Unicode characters that look like it mean?
And bonus points, how do I properly insert that character on Mac?
Here are some candidates:
× U+00D7 Multiplication Sign
✕ U+2715 Multiplication X
U+2715 (“Multiplikation X”) is in the Dingbats block. Therefore, it’s for ornamental usage (if anything) and not for communicating mathematical relations. There is no reason to expect that any font renders it in a way that would be appropriate for a mathematical operator, in particular I would expect it to be to bold and large for this purpose. Using it for normal mathematics is like using U+2757 (❗, heavy exclamation mark symbol) instead of a regular exclamation mark.
By contrast U+00D7 (×) is intended for multiplication as indicated by an explicit multiplication sign missing from the Mathematical Operators block. You can expect it to be properly aligned with the numbers and match other mathematical operators in size and style.