You can only network helix to helix and Solix to Solix for one answer. Only helix to helix networked, you can share is waypoints, Solix to Solix networked, you can share maps and waypoints. Mega 360 has to be plugged into the transducer port of helix of which if you were getting a 9 Mega DI+ or SI+ G3N then plug it in and if networked to other helix models your mega 360 is viewable.
if Using a Y cable, the 9 M360 2DDI Y, you can plug the Y cable into your 9 mega plus G3N DI+ or SI+ Then plug the Mega 360 into one side of the Y and your transducer to the helix into the other side. If using a SI model transducer plugged in, the Y cable will only show DI and 2d and you will have to use the Mega 360 for viewing SI so you would save money buying a 9 Mega DI+ G3N. If you have a trolling motor with a DI or 2d built in transducer, you could plug it into the Y cable and mega 350 to the other side if the Y. You have that option. Reason for using a Y cable is to get a depth reading on the mega 360. If networked, you could get the depth reading off other unit networked.

Here is what I have. I have a 9 mega SI g2n on the console and transducer on the back of my boat. I have a 7 DI mega g3n up front for 2d, DI if I want it, and charts (maps). I have a 10 Mega DI+ g3n with the Y cable and the 10’s transducer plugged in and the Mega 360 and all of it networked. I have no transducer on the 7 and can get all depth, temp, 2d, DI off the 10. Even Si off the 9 or SI off the mega 360 as all you do is change network settings. I can if I wanted to, see and use mega 360 on all 3. Just remember that on the 7 because I have no transducer plugged in, go into your settings and turn “sonar settings “ OFF.