A groove, not a bar
A keyway and a dowel do the same job by different means, and that is usually why the two words show up in the same breath. A keyway is a physical shape, a groove pressed or formed into the wet concrete along the top of a footing, so that when the wall goes on top of it, the hardened concrete has a ridge locked into a channel instead of one flat surface sitting on another. A dowel does the same connecting work with steel instead of shape, a bar left standing up out of the footing and tied to the wall's vertical steel once the wall gets formed. Either one keeps the wall from sliding sideways off the footing under lateral pressure, and a job typically gets one or the other rather than needing both.
Formed while the footing is still soft
The groove goes in during the footing pour, not after. A beveled strip is set into the wet concrete along the centerline of the footing and screeded to, or a formed key insert does the same job, and it comes out or stays embedded once the footing has taken its shape. Miss that window and the footing sets with a flat top, and there is no cutting a keyway into cured concrete afterward the way there is no drilling a hook bolt into a wall that has already hardened. At that point the fix is dowels drilled and epoxied into the footing instead, which is a different scope than what was planned.
Which one a footing gets
That call comes off the plan, not off habit. Some engineers spec a keyway, some spec dowels, and some jobs get a straightforward flat footing with nothing more than the vertical steel tying into it because the load and the site do not call for either. We build what the drawings call for, and if there is no engineered plan yet, tell us what is going on top of the wall and we will tell you what the footing needs.
Why the wall needs to lock in at all
A footing and the wall poured on top of it are two separate pours, hardened concrete meeting fresh concrete rather than one continuous piece. Left with nothing but flat concrete under it, a wall relies on nothing but its own weight and a bit of bond to stay put sideways, and that is not much to lean on once backfill starts pushing against one face of it. A keyway gives the two pours a physical shape to key into, so the wall has to shear through the ridge of concrete rather than simply slide off a flat top. That is the whole point of either a keyway or a dowel, and why a footing without one is usually not what the drawings intended.