To use the Mirror tool inside modeler
Multiply > Modeler > Mirror or press shift (f)
If you would like to use the Mirror tool, the first thing to do is to choose the Modeler option from the Multiply menu. Later, you need to select the Mirror option under the Duplicate or press the shift (v) to select the mirror tool. It’s used to clone the object on the other side just like your mirror.
The function of the mirror tool is to clone one object to another.