How do I save my photos but delete them from my phone

Where are you wanting to save them - you have many options.

If you have a mac, just airdrop them over, you can share the ones you want & you can also upload them to photos app on a mac.

You have dropbox and box and a host of other third party places to store them as well as iCloud.

What are you trying to do exactly?

You can use iCloud photos to backup the photos and then check optimise storage is turned on. This way only recent and most viewed images are kept on your iPhone at full resolution and the rest are kept as thumbnails. If you want to completely delete photos on your phone and don’t want to pay for iCloud you can use google photos which gives you unlimited storage at high resolution which is a bit less than the original quality of the image but it’s hard to notice any difference.