How Google Trip Bundle works? Google Trip is now performing automated clustering of travel items into trip bundles. 

https://support.google.com/inbox/answer/6228360?hl=en-GB

So, if in your mail you have a flight confirmation mail and a hotel confirmation mail related to the same trip, Google will cluster them automatically into a trip bundle.  

This means that that:


They scan all your e mail
They find the ones related to trips
They cluster those for the same trip into a bundle.


My question regards point 3. Which kind of algorithm is needed to perform such task?

Consider that the task is unsupervised, because the user can not interact with the system while creating the bundles..
Company secrets - Google does not disclose what they actually use. Any answer to this is pure speculation.

It probably is not even just "one" algorithm, but many. But it may be as simple as finding overlapping dates and locations, assuming that people cannot be in two places at once. You could test this: book a flight, and a hotel on a different destination during the same time interval. I wouldn't be surprised if they nevertheless show up as one "trip" (and it doesn't really degrade user experience much).

It will very likely be a supervised approach. Which does not mean "interactive". On the contrary, unsupervised approaches will likely need interactions to choose parameters. Using some training data (e.g., from employees, or user interactions) to improve matching quality looks promising, though.