OnX and Google Earth provide some different things between each of them. They each have their own advantages and disadvantages. One cool thing is you can input pins or tracks into Google Earth and then upload them to OnX so you have them on your phone.
Two separate tools that are both helpful in my opinion.
It's surprisingly not too difficult. Once you have something in google earth, you right click on the icon for it in the left hand toolbar under 'Places' and click 'Save Place As'. Save it somewhere you'll remember, like your desktop.
Then, open up a browsing window and go to onxmaps.com. Log in and then click 'My Content' on the left hand side. Click the 'Import File' button, then click 'Select From Computer'. Find the item, wherever you saved it, and click 'Open'. Then click 'Save to My Content' and you are good to go. It will now show up on your phone under 'My Content' in the OnX App.