Create a document say 1000 x 500, you need to use the 1000 and divide by 2 and create a rectangle that is 500 x 500. Set the color for the rectangle and then add some additional paths to it such as circles and text etc.
Drag the entire set into the symbols panel (which is found via the window menu). https://youtu.be/aN54FYC9fPs
Once you have this, go to the effect and distort and transform category and transform command. Use the width as the value for the move horizontal (say 500) and then reflect X on and set the copies to 1 and so now you have a live transform of the design but as it is an instance, you can edit it and change it all kinds of ways and it will still keep the effect. The transform effect can be used for countless other things such as zooms, grids etc https://youtu.be/ft3E5oYFz4E
Double click the entry in the panel and add additional paths, remove paths, change colors, add strokes etc and then close the instance and now because of the live effect and also the preset, you will have a mirror effect applied to the changes you made to your paths https://youtu.be/soBVOsNHYiU
You can repeat this at any point. It should be noted that there is now a repeat feature in the application that can also be used with normal paths as well so this approach can still be used, you may find the repeat feature even more useful. It is easy to set up and apply via the object menu with any selected paths https://youtu.be/Ike0J6csauY