iCloud Photo back up confusion

I am so confused at the functions icloud photo library has.

So we can upload it to icloud - back up in settings. Then you delete the photos off your phone to clear up space, but by doing so this deletes off the icloud AS WELL. HOW do i go around this???

I read from another google search that to turn off your icloud before deleting them.

What happens when you want to sign back into your icloud, will it sync again and delete the pictures you've deleted?

I don't understand the point of having large storage phones when there is no easy way to store/upload/keep your media.

Only reason I'm coming across this is because I need to. back up my phone before sending out for repairs - which my phone needs to be reformatted to factory settings before sending it out.