Typically, model railroad benchwork (the "table") is built by the individual hobbyist to meet the needs of one's particular layout. Since, as a general rule, every model railroad is different (or can be different), benchwork is built to suit the needs of the individual.
The simplest is "tabletop" construction--typically a rectangular framework of 1x4 boards with a sheet of plywood screwed onto it. This is the flat "table" you may have seen. Its advantages are simplicity and ease of construction, plus the ability to redesign one's track plan on the fly--disadvantages are heavy weight and flat profile (you really can't model bridges, ravines and mountains on such a layout.)
An older but still common technique is "cookie-cutter" construction: A frame with regularly-spaced crossbraces is built, the plywood sheet is cut to closely match the outside dimensions of the track plan, and wooden risers hold the "cookie-cutter" plywood in place above the frame. Scenery is modeled in between the gaps in the "cookie-cutter" platform using techniques like hardshell (plaster-soaked paper towels over a cardboard-strip or balled-newspaper form) or foam or gauze & plaster.
The frame can be a simple rectangular frame or a special lightweight framework known as "L-Girder" benchwork. The advantage here is that grades (slopes up and down) can be easily modeled, and since you're not using the cut-out areas, weight is much lighter than a traditional tabletop, and you lose that "children's play table" look.
In the past decade or two, the use of extruded foam insulation sheets (and other foam-based products) for tabletops and scenery has become much more popular. While it takes a little practice to work with, foam sheets on top of a lightweight wooden framework can provide a lightweight, carveable tabletop surface that can be utilized as a flat surface (like the basic tabletop) but gullies can be carved below grade or mountains stacked above (like cookie-cutter.) There are some other tricks to foam--my advice is to read up on benchwork (either online, at your local library, or your local hobby shop) to get specific details on the how & why of benchwork.
But, generally, you'll want to build your own. That way you'll get exactly the benchwork that you want.