At each corner of your vehicle, a wheel bearing is spinning around furiously when your car rolls. The wheel bearing is the part that enables the rotation of the wheels. It is located in the center of the wheel assembly. It consists of a housing and internal roller or ball bearings that are in what are called races, or holders. There is a lubricant in there as well. That lubricant is “for life.” You don’t “lube” a modern wheel bearing. They are also not serviceable in the sense that you can remove, clean, lube, and re-install them. You replace them.
The term wheel bearing is not one that has a single meaning. Many shops will say, “You need a wheel bearing,” and what they mean is that you need a wheel bearing assembly that they source as a single part. It includes the housing and the internal bearing itself. Sometimes it is also part of the wheel hub assembly. This is not always a bad thing, by the way.
If your vehicle uses a bearing that can be replaced, the shop will remove the housing, take out the old bearing with a press or by other means, and then put in your new wheel bearing using a press. That takes time. Time is money. Pressing in a wheel bearing is not vaccine science, but it's also not usually a DIY job. If the bearing comes as part of an assembly, it may actually be easier to install and cost just about the same. So don’t panic if you hear “assembly” as part of the job description.