how to remove photos from iPhone 8+ without deleting from other devices

Go to Settings/[your name]/iCloud and turn off Photos. You can then delete them. However, you can never turn it back on again, or all of the photos will come back from iCloud.

Another option is to turn on Optimize iPhone Storage; this will keep the photos, but in a smaller, lower resolution format. The photo storage will be reduced to 1/4 of its original size.

Parallel question; I want to delete photos from my phone, but not my computer; phone says, "this photo will be deleted from the iClould Photos on all yyou devices." Will it take the photos off my laptop too? I want to save photos on the computer, but not on the phone. How do I do this? Thank you

iCloud is a syncing service, so if you sync your laptop to iCloud then when you delete photos from your phone they will be deleted from iCloud and also from the Photos app on your Mac or from iCloud for Windows on your PC. But on a computer you have a way around this: export the photos to a folder on your computer from Mac photos or iCloud for Windows. These exported photos will then not be synced to iCloud, so they will be safe.

It’s also a good idea to archive photos. This is not the same as syncing, and cannot use iCloud. You need a 3rd party archiving service; examples include Google Photos, Dropbox (which can archive any files, not just photos), box.com (same idea as Dropbox), Amazon Photos, Shutterfly, Livedrive, and many others.