if I delete all contacts from one device, will they still be in the cloud and on my other devices?

If you disable contacts on that one device in your iCloud account settings, you will get a prompt to either keep the contacts on the device or erase them. If you erase them, it will only erase them on that one device. All other devices connected to your iCloud account and using iCloud for contacts will continue to sync contacts.

But if you delete contacts while a device is sync’ing contacts with iCloud, you will delete them everywhere.

173emberlane wrote:

If I delete all contacts from one device, will they still be on my other device and in the cloud?

Not if they are being synced via iCloud. That is each device is signed in with the same Apple ID and contacts is selected in the iCloud settings.