First, decide where you want to keep your collection of photos and videos. You can make them available on all your connected devices with iCloud Photo Library, or you can choose to store them locally (only on your Mac or PC).
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iCloud Photo Library
With iCloud Photo Library, you can access your photos and videos from your iPhone, iPad, iPod touch, Mac, Apple TV, on iCloud, and even your PC. Your latest shots are automatically added to iCloud Photo Library, and any organizational changes or edits you make are always kept up to date across all your devices.
Before you begin:
The photos and videos that you keep in iCloud Photo Library use your iCloud storage. Before you turn on iCloud Photo Library, make sure that you have enough space in iCloud to store your entire collection. You can see how much space you need and then upgrade your storage plan if necessary.
Turn on iCloud Photo Library:
If you've already synced photos to your iOS device from iTunes, and then you turn on iCloud Photo Library on your iOS device, you'll see a message that says "Photos and Videos Synced from iTunes will be Removed." The photos and videos that you synced from your computer will stay on your computer, but they're removed from your iOS device.
You can get these photos and videos back onto your iOS device by turning on iCloud Photo Library on your Mac or PC. When you do that, the photos from your computer will upload to iCloud so that you can access them on all of your devices. After you turn on iCloud Photo Library, all photos are in the cloud and accessible from the Photos app on your Mac or a folder on your PC.
You can get more help using iCloud Photo Library on your Mac or PC.
Import to your Mac
You can use Photos for OS X to import photos from your iOS device to your Mac without using iCloud Photo Library. These steps also work for importing images from digital cameras and SD cards.
Imported photos appear in the Photos app's Last Import album.
Photos and videos that you sync from your computer to your iOS device using iTunes can't be imported back to your computer.
Learn what to do if you can't import photos from your iPhone, iPad, or iPod touch to your computer.
Import to your PC
You can import photos to your PC by connecting your device to your computer and using Windows Photo Gallery:
Then, follow the steps in these Microsoft knowledge base articles to learn how to import photos to Windows Photo Gallery:
When you import videos from your iOS device to your PC, some might be rotated incorrectly in Windows Photo Gallery. You can add these videos to iTunes to play them in the correct orientation.
Photos and videos that you sync from your computer to your iOS device using iTunes can't be imported back to your computer.
Learn what to do if you can‘t import photos from your iPhone, iPad, or iPod touch to your computer.
Get photos from your computer to your iOS device
You can choose among several options for transferring photos and videos from your computer to your iPhone, iPad, or iPod touch:
Create a folder in iCloud Photo Sharing, and add pictures to it you want to keep, once they are in the shared folder, you can delete them from your phone/iCloud photos. They will remain in the iCloud Photo sharing folder regardless if they are deleted from you phone.
I use this for groups of pictures for other peoples stuff I don't want in my iCloud.
Example: took a bunch of photos of a car that's not mine, added it to the Shared iCloud folder and people to share it with or don't add them and then delete them from my “photos” and the shared folder remains for anyone you want to share it with. The secret is that if you do it this way it doesn't count against your iCloud storage.
You can try Safewiper iOS Data Eraser to manage your iPhone via computer.
It can scan all "already deleted" and "existing data" from your iPhone, then completely erase and never recovery.
I record a lot of videos; eating into the 64gb's on my iPhone, I remove them from the device once I'm near my iMac and transfer them to my iCloud Drive. That way they're still in the cloud, they're still backed up locally (on the iMac and through Time Machine to an external drive), it just takes some effort rather than being automatic.
Personally I'd like to see iCloud Photo Library keep the last 30 days of photos and videos on a device, keeping the rest in the cloud. If you want to delete anything them you go into the cloud to remove it. It would be far simpler and more straightforward.