TripAdvisor was owned by Expedia from 2004-2011

It's still part of the Expedia portfolio. There are a few brands whose stocks are listed separately.

What percentage of TripAdvisor does Expedia still own? The stock looks to be mostly held by various mutual funds although I didn't look very closely and may be missing something.

That's just accounting. Doesn't matter. Businesses are not run independently, they share some employees and services.

When you join the company, they will explain you the portfolio of brands and how they link together. A user might leave a site and spend money somewhere else, but it's alright as long as it stays in the family.

They're independently owned. You could argue that it was just accounting if the two businesses had the same or similar ownership while being two separate entities, but they're two different publicly traded companies with substantially different ownership. They also don't share a single board member. Can you provide some sort of substantive indication that they're operating together (today, as compared to prior to the TripAdvisor spinoff)?

For context, here's where you can see the substantial differences in ownership: https://finance.yahoo.com/quote/TRIP/holders/ vs https://finance.yahoo.com/quote/EXPE/holders/ .

And here's where you can see that they share no board members: http://ir.tripadvisor.com/corporate-governance/board-of-dire... vs https://www.expediagroup.com/about/leadership/ .

Join the company and you will see for yourself. I'm not going to explain how a business empire works over a chat message.