How to delete files/photos one by one from iCloud?

If you go to www.icloud.com and sign in with your appleID, you can open photos there and delete the ones you don't need anymore. If you want to delete them immediately , delete them also from 'recently deleted'

If you use iCloud photos on your Mac, iPad and iPhone, you can also delete them there. iCloud is a syncing service

See the article https://support.apple.com/en-in/guide/icloud/mm3b7fcd0c10/icloud

https://support.apple.com/en-in/guide/system-information/syspf5a64aa6/mac

https://support.apple.com/en-in/guide/icloud/mm08b49040b5/icloud

For a single file / photo tap it and click on delete , hold shift key and tap the photos / files to get selected , and then delete them .

Select by clicking on the photo and delete