okay so i want to switch everything from one iphone to another without starting from new. aka old to old without wiping out data. how is that done?

You cannot do so. If you restore one from the other's iCloud or iTunes backup, you inherently erase the content already on the device being restored.

Quickstart will also do the same thing when using two already setup, activated and in use device - you would end up replacing the contents of one with those of the other.

None of the setup, restore or migration tools are designed to merge contents of two already setup and activated devices. They are all intended to migrate contents from an existing device to a blank, new (or recently erased) device.

If both iphones are running iOS 12.4 or later you can use this migration procedure:--> Use Quick Start to transfer data from your previous iOS device to your new iPhone, iPad, or iPod touch - Apple Support

Regards

Giulio