Why is he saying ”This is us” when it means “This is ours”?

It's commonly used when in a queue-like situation to show that it's our turn. In a lift, for example, when you arrive at your floor, it would be acceptable to say 'this is us'. In the second example, I would be more likely to say 'this is ours', but it's similar. The sites are allocated, so there's sort of a queue, but it's a stretch.

You will commonly hear someone (in both North America and the UK) say "this is me" when either riding a bus with someone else or walking together. It means something like "this is my stop" or "this is where my path diverges from yours" or "we've reached my destination, so I'm stopping now". It's an idiom, which means it doesn't have to make sense. I hadn't heard "this is us" before, but to me it's just an obvious pluralization of "this is me."