Would I need a usb and/or the computers to be side by side in order to transfer my characters? I want to transfer my characters from my mac to my pc.
You can use a cloud service like Dropbox to backup your starbound saves folder.
I would suggest not replacing the universe saves from mac to windows or viceversa because they are not compatible due to the universe seed being different.
By moving your character home you should be able to access and use it.
As your world is not moved, you will need to store what you want to bring over in your inventory. Or try to copy also the ship files