How do you read the symbol “x” when used instead of a preposition?

Solution 1:

The points of the compass do not have a "South by Southwest" and the "by" is abbreviated "b".

Moving clockwise from South, we have:

south by west (SbW); south southwest (SSW), southwest by south (SWbS), southwest (SW). See Wiki at 32 Compass Points

As far as "Adidas x Stella McCartney" is concerned, I found this on the Harvey Nichols site. I would read it as ""Adidas designed by Stella McCartney"

There is also the biological use to indicate a hybrid, e.g. a liger is Panthera tigris × Panthera leo. Here, 'x' = crossed[bred] with.

"X" is only understandable in context. There is no rule (but I think you already knew that.)