system storage full after migrating to new iPhone, old phone has plenty of free storage I have an iPhone 11 128GB with around 30GB of used storage.
Most of the photos are uploaded up into 200GB iCloud which is full at the moment.
I got an iPhone 12 and transferred data &amp; settings over from iPhone 11, device to device as described here https://support.apple.com/en-us/HT210216. The transfer itself took around ~30min.
What I ended up with, is an iPhone 12 with 122GB of storage is use, photo library taking around 90GB of it. My assumption is that for whatever reason a LOT of the photos from iCloud got stored locally on iPhone 12. The phones have same amount of photos in the libraries.
How can I get rid of the photos that that are currently LOCAL on iPhone 12, while they are living in the iCloud (uploaded from iPhone 11)?
You most likely have selected keep originals in the photos settings.
Open the settings app and scroll down or search for photos. Then under the iCloud section there should be listed the following options:

Optimise iPhone storage
Download and keep originals

Select the first option and your iPhone will offload older photos and only store them in iCloud. Only new photos and often viewed ones will be stored locally, so you can save a lot of space on your phone. Alternatively you can select this option if you go into [your name] -&gt; iCloud -&gt; Photos as described on AppleSupport.