Dumb question, hoping someone knows

[Locane]

When two companies merge, what happens to the stock ticker data?

For example; ATVI is the ticker for Activision / Blizzard, but
Activision and Blizzard didn't merge until late 2008.

What exactly am I looking at with Google's price data before that
period? Is it an average of the two? Is it just one? Is there a way
to separate them?

Being very new to all this, I couldn't find the answer immediately
available online.

[ABCDE]

Blizzard was never a public company on its own. It was a subsidiary of another private company, which got absorbed into Activision, which was publicly traded prior to that acquisition.

The (very general and oversimplified) heuristic though is that the acquired company's stock price rises, while the stock price of the purchaser decreases.