The only way bike to bike intercom systems can communicate is via a wireless signal. There are four types of radio technologies used in the U.S. for motorcycle intercoms. They are GMRS, FRS, FM, and Bluetooth.
There are benefits and disadvantages of all of these systems and the best one for you really depends on how and where you drive. If you drive mainly in congested areas you would likely want to pick Bluetooth. If you need to communicate over long distances, then GRMS would be best.
You can read a detailed article about choosing a bike to bike intercom by clicking here: Motorcycle Intercoms – How To Choose The Best Motorcycle Intercom For You