1. If you delete a photo from your iPhone (or any other device you’re using iCloud Photo Library with) it is moved to a recently deleted folder. It will remain there for 30 days, after which time it is deleted forever and is no longer available on any device or in iCloud Photo Library.
You can’t delete them from your device but not iCloud, it doesn’t really work that way. If you ever find yourself short on space, just enable optimised storage and let the library figure out when it needs to remove the full quality images from your device to free up space.
2. There isn’t really a master device with iCloud Photo Library. The best way to use it is to enable iCloud Photo Library on all of your devices and forget about it. Any changes you make on one device are reflected on all other devices.
If you were to buy a new phone, you just enable iCloud Photo Library on it and all of your photos are synced across to it from the cloud.
If you had taken some photos before enabling iCloud Photo Library, you’re given the option to merge those into your iCloud library - iirc, it’s been many years since I didn’t use iCloud Photo Library.
Essentially, the cloud itself becomes the master device if you like, all of your photos, in their full quality are uploaded to it and then synced between devices from there and as I mentioned, any change on one device, gets filtered down to the rest of them.
Even if you’re not using optimised storage on your iPhone (or any other device) it works in exactly the same way. The only difference being that with optimised storage disabled, all of your photos are saved in full quality on your device as well as in the cloud, to save you from having to download the full quality image before making edits, or suchlike.
