>Don't most American houses have at least one 240V socket in the garage, for use with garden/garage equipment?

Newer ones, depending on location.

>(Otherwise, where do you plug in an electric car if you're visiting relatives, for example?)

You charge on 120v or you don't charge.

No. Most US houses have, at most, a single 240V outlet in the laundry room for an electric dryer. Literally everything else is 120V. There is almost no household equipment other than drying machines that is sold taking a 240V plug.

Electric cars may change this. They haven't yet, at least not much.

You forgot the outlet for the electric range, so 2 outlets for most US homes.

Ah, you're right. Doubled! :D

(The important thing to note for non-US-dwelling folks is that your average American simply never deals with plugs other than 120V except for rare circumstance, like appliance replacement or adding a car charging portal.)