How a house is built
How wide the concrete is
A footing is wider than the wall so the same load presses the soil more gently. This sketch does not size one.
Arrow keys turn the house. Plus and minus, or the zoom buttons, change the zoom. A horizontal drag turns it after you tap the drawing. Reset view returns to the step camera.
The drawing needs WebGL. The notes on the page still stand.
Soil can take only so much pressure. Spreading the load over a wider footing keeps that pressure down. A continuous footing follows a bearing wall. An isolated footing sits under a post. The bottom has to reach the depth the local code sets for frost, unless the design is a frost-protected slab, which is its own assembly.
The highlighted base of this house is the idea of that line. Bearing pressure, reinforcement, and thickness are calculated from the loads and the soil. None of those numbers are in the drawing.