I clicked on my Car first, added Boolean modifier, then eye dropper selected the circle, made sure difference and exact were selected like in the example and my care dissapears.
Hi Xavier, I can't find the moment in the video, you are talking about, but
Circles do not make good Boolean Cutters, but you probably mean Cylinder, or so. And maybe you need Fast in your case, if Exact doesn't work. Also possible that you have inconsistent Normals, or some other problem, hard to tell with so little information.
Yeah that's something that booleans do, don't worry, nothing weird has happened. Booleans are finicky by nature, you just have to wiggle the object around a bit or change the solver from fast to exact or vice versa. As Martin said also check if you have flipped normals and recalculate them.