I've used the mirror modifier in order to create a kind of speaker (for the exercise) and it worked. But when I want to move it, the mirrored one moves in the same direction. It should be the opposite, no? Also, I note that that mirrored object is selected (in orange) when it shouldn't. And the gizmo is between the two objects, while in the video it is on the principal one. What is my problem???
The mirror modifier uses the object's origin point (the yellow dot that appears when you select your object) as the point from where to start mirroring from. If you move your object in object mode, the origin point will come along and the mirrored side will come with it. If you move your object in edit mode, the origin point doesn't come along and the mirrored side will act accordingly, or in other words, the moving in the opposite way behavior like you were expecting. The gizmo will be displayed wherever your origin point is.
If you want to watch the full list of modifiers explained, you can watch the Modify course:
That's logical. I don't know what I missed with Jonathan Lampel instructions (he is excellent but speaks very fast for a French speaking person!) but now, it works. Thank you and I will watch the Modify course for sure!