In most cities, the short answer is: urban trash bins are typically emptied from the top. Standard street bins and wheelie bins are designed with lids on top, and collection trucks usually lift the bin, open or tip the lid, and let the waste fall out from above. That is why most public bins and household wheelie bins have handles, wheels, and lids arranged for top-loading collection.
You will also see this with larger dumpsters. Front-loading garbage trucks lift the container and tip it over the top of the truck, so the waste pours out through the opening. Rear-loading trucks work differently on the truck side, but many of the bins themselves are still tipped so that the contents come out from the top.
There are exceptions. Some underground or semi-underground waste systems, especially in dense urban areas, may use bottom-access containers or vacuum collection systems. Specialized trucks or equipment may empty these from below or through a lower hatch. But for ordinary urban trash bins on sidewalks, in parks, and outside homes, top emptying is the norm. So if you see a garbage truck lifting a bin and tipping it into the truck, it is almost certainly emptying it from the top.