Parent quote provided a very accurate summary. There's actually nothing of substance in that quote, and it certainly doesn't answer the question: what does Sidewalk Labs actually do?
> we launched Sidewalk with the broad belief that if we combined the best in urban planning and design with the latest in technology and open digital infrastructure, we could meaningfully improve quality of life for people in cities [...] That’s what we hope to achieve with Waterfront Toronto: neighborhoods that pilot ideas, new and old, to solve big urban problems that cities everywhere are facing. [...] We see our role with Waterfront Toronto as trying to put in place some of the underlying infrastructure [...] On top of those systems, all kinds of local innovators, civic groups, academics, etc, will fill in the details will their ideas and creations.
How is this meaningfully different from what I said? Suppose we rewound time to before cloud infrastructure like AWS and Google Cloud existed, and Google came out with App Engine saying, "we put in place the underlying infrastructure so people can build useful things on it".
And people just keep asking, "but what does it do?" In both cases, the answer seems to be that it provides a platform for other people to implement their own innovative ideas. Given it's Google, this neighbourhood will probably provide ubiquitous wifi and power availability, fiber internet everywhere, and the "livable city" constraint means it will be a walkable space, so few cars, if any. I'm sure there will be plenty of other useful infrastructure I haven't even thought of.
Those are two different things.
Back to the original question: Is it, for the most part, a consulting firm? A developer? A construction company?
They would act kind of like a developer here, but it's not to develop specific things, like commercial or residential space, it's probably a lot more open than that, which is why people are having trouble classifying it using traditional terms.
It's like trying to classify App Engine or AWS before cloud infrastructure became a thing, when everyone ran their own servers; it requires a new category all its own.