Mirror a single object:

You just select your object and choose one of the Mirror planes (X,Y or Z).

The mirror plane will be added at the origin of your object and by moving the Mirror plane on the axis, your mirrored object will be showing.

You can also do the rotation if you like.

When you apply the mirror, you have an individual mirrored copy of your object.

Mirror multiple objects:

Select your objects and join them with Ctrl J.

Choose one of the Mirror planes and hit apply.

The original and newly mirrored object will now be all seperated by loose parts and will all be individual objects.

Use clipping if you want to have your mirrored copy and the original object as a 1 object result.

The origin will be in the center where the Mirror plane was.