Q: If I delete all photos from my phone, will they stay in Icloud

iCloud Photo Library is for syncing.

If you delete a photo from your iPhone, it will be deleted from iCloud and all other devices syncing with your iCloud Photo Library.

To free storage on your iPhone use the option "Optimize Storage".  Then you iPhone will only download smaller, optimized versions from iCloud and keep the full resolution originals in iCloud.

iCloud Photo Library FAQ - Apple Support