The data is grabbed in real time when you search for a city. For this reason, it might take a few seconds for the buildings to show up on the map. Also, considering the insane amount of data that needs to be downloaded in a few seconds, attempting to view large cities at a low zoom level, might cause your browser to freeze for a few seconds and in some cases to crash completely. If it does crash, you can retry to view at a higher zoom level by adjusting the bar and trying again.
Via spatialcarto & urbanfabric