Which automobiles share platforms?

The Audi A3 and A4 share many things in common with the VW Jetta/Golf and Passat. It’s not a 1-to-1 relationship, and all 4 cars have some shared engineering elements.

When these auto-makers platform share, is everything done at the same factory? Or are multiple factories used, utilizing the same plans to produce?

I’m not sure about which factories are used, but I happened upon news that Rovers might be built again in England, but shipped first in a semi-manufactured state from China first.

Platform sharing may be in the same factory, or it may be in other factories.
Example the Volvo/Ford platform share I mentioned in my post above. The Volvos using our P2 (internal designation) are built in Gothenburg, Sweden and Ghent Beligum. The Ford versions of this platform are built in North America.
On the other hand the first version of the Volvo S40 (2000-2004) was built in a factory where it shared a platfrom AND an assembly line with the Mitsubishi Carisma.
Platform sharing does not mean they are the same car. The S40/Carisma were not the same car with different name badges. If you parked the two side by side, you would be hard pressed to tell they shared anything. Furthermore if you saw them on the assembly line before the fenders were bolted on, you would know right away which one was a Volvo and which was the Mitsu. (crash members)
Platform means the base structure and where and how some attachments are made. It does not mean the same car with a different name badge (but it can)
When I look at a Ford 500, I can see a few things that are lifted directly from our P2, and some that are adapted, and some that are all Ford.
The advantage of platfrom sharing is that the car company can save millions, maybe billions of dollars in development. In the case of Ford they took a platform that had been in service since 1999, and put in their car. Saved lots of bucks and built the best Ford ever. (IMHO of course)

If you paid $13,000 for a Yugo, you got SCREWED! I believe they had a new sticker price of $4995.00 20 years ago.