The easy way to think about U.S. airspace is that usually there are one to four basic layers of the ATC cake, plus chunks of other goodness.

Non-radar Towers (over 300 stand alone towers) - Control the immediate area of the airport, up to a few thousand feet.

Approach Controls/Radar Towers (over 150 radar equipped in the U.S.) - Control larger geographic areas around one or more airports, and their airspace height varies from several thousand feet to, in a few cases, over twenty thousand feet.

EnRoute Centers (22 in the U.S.) Control large areas of the country, and have the layers mentioned at the beginning that abut or overlay the other facilities. Depending upon the area, a Center may have little airspace down to the ground (New York, Boston, etc) or a lot (Denver, Fort Worth, Minneapolis, etc).

Within EnRoute, you usually have Low Altitude and High Altitude, often split at FL230 (23,000 feet, corrected for air pressure…the correction, in the U.S., occurs at 18,000 ft/FL180). There can also be Intermediate and Ultra High sectors in locations where there is a lot of traffic, all designed to more evenly split the workloads.

This is the basis for “unusual” altitudes. You’ll see flights staying at or below 23,000 ft to avoid a congested High Altitude sector/facility…or, in this case, as explained above, at or below altitudes that allow it remain within only Approach Controls to avoid Traffic Management restriction of flights entering the overlying Centers. This is all heavily simplified.