I was part of "Consumer Psychology Team" at Booking.com for years. That team was/is one way or another involved in pretty much every functionality you mentioned (among other things I did in particular latest version/implementation of "X other people looking at this") and TL;DR: All of it is real - and majority of people don't find it as annoying as you, many even find it helpful

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While I can't really give too many details (BTW I still work at Booking - just moved teams).

I did the real time (it's a stream so technically that's "near real time") processing back-end which collects all the "views events", uses anonymous unique user identifiers to make sure it's truly not counting duplicates, bots are also excluded ...etc. And result is a reverse index of sorts.

On the FE side (technically still BE so think backend for frontend) as part of producing message (or not) we literally do a real-time query on that reversed index to be sure that even if looking only at count it's "1" - we want to be sure that it's not you.

As of things like "Only X left" - that's literally how many rooms we've got left to offer to you for the dates you've entered.

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And speaking generally. Booking.com being 25+ years old global company, one can imagine that company is basically constantly under microscope of various regulators and consumer protection agencies. So even if there was someone that wanted to fake this type of stuff (and that would be going against what they've learned in various mandatory ethics, privacy, legal ... training). Well something like that would've been very short lived.

And with so much money on the line - companies (at least those that don't have walled garden ecosystems and such) actually do look into long term effect of things like Marketing/Sales/Persuasion - because they don't want to annoy potential customers so much that they never come back.