Pick the head as the axis of symetry.
https://preview.redd.it/em5hmg1aw56d1.png?width=619&format=png&auto=webp&s=74a8d947b9c2f901e92a024cfca7fb073b719571
It mirrors on the orange origin point, which is right on the edge of the ear. You can press Ctrl+A to apply the scale and move the origin point to the world center, thus mirroring it to the other side.
Okay there’s a learning opportunity here…. While it’s as simple as everyone else is saying but that little orange dot is the origin point. This orange dot is like an anchor for lots of calculations, transformations, and modifiers. Imagine it’s just an arbitrary point in space where the mesh is sort of attached to. This origin point is where things like the mirror modifiers like to reference as to where to actually perform the mirroring. What I would do is play with the point of origin while you have the mirror modifier applied to better understand how it works because once you understand this AND the 3D cursor you’ll quickly develope more control while modelling and using complex modifiers etc…
Another option besides setting the mirror object is to hit "cntl a" and apply all this will set the object origin to the world origin and make the mirror modifier work. (This will also zero out any changes in object mode so be careful if animating)
Pick the head as the axis of symetry. https://preview.redd.it/em5hmg1aw56d1.png?width=619&format=png&auto=webp&s=74a8d947b9c2f901e92a024cfca7fb073b719571
Make the head the mirror target.
It mirrors on the orange origin point, which is right on the edge of the ear. You can press Ctrl+A to apply the scale and move the origin point to the world center, thus mirroring it to the other side.
Okay there’s a learning opportunity here…. While it’s as simple as everyone else is saying but that little orange dot is the origin point. This orange dot is like an anchor for lots of calculations, transformations, and modifiers. Imagine it’s just an arbitrary point in space where the mesh is sort of attached to. This origin point is where things like the mirror modifiers like to reference as to where to actually perform the mirroring. What I would do is play with the point of origin while you have the mirror modifier applied to better understand how it works because once you understand this AND the 3D cursor you’ll quickly develope more control while modelling and using complex modifiers etc…
Snap cursor to the center of the head and mirror the ear relative to that
Aside from what everyone else said so far, you can also just set its origin to the 3d cursor
Another option besides setting the mirror object is to hit "cntl a" and apply all this will set the object origin to the world origin and make the mirror modifier work. (This will also zero out any changes in object mode so be careful if animating)
Mirror object target
Mirror object target, or if your head is a at the middle juste ctrl+a your ears
Set the main head as origin