I did what was taught in this tutorial:
I selected the bones on the side that I made, right-clicked and selected the symmetrize option:
But it did not work:
I tried to do the bones on both sides at once, but I didn't know how to do that in this version of blender.
If I'm not mistaken in previous versions, it was just extruding using shift/control (I'm not sure which of the keys exactly) + E.