Hi Dean,
Re: will they stay on my iPhone ?
No.
Seems iCloud is mainly for sync of data across devices
ie .... delete from iCloud or any one device and that syncs across to delete the same from all in sync.
Also: If your iPhone is on Low Power Mode, you may only have low-resolution photos on it.
So you would need to download your original high resolution images from iCloud.
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To safely save everything currently in iCloud photos,
or to keep copies in case you find you do need any of the pics:
How to: Download iCloud photos and videos – Apple Support
or
How to: Archive or make copies of the information you store in iCloud - Apple Support
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If you want to, and if you are sure you have downloaded / saved everything you wish to keep safe
you can turn off sync of Photos to iCloud.
Eg: On iPhone or iPad:
Go to Settings -> tap your Profile -> tap iCloud -> scroll to Photos -> turn it Off.
More info: Get help with iCloud Photos – Apple Support
All the best :-)