Restoring backup

Hello my iPhone 8 Plus is gone away to apple for repair so I backed it up on my phone before I gave it to them , I’ve gone to log into my Apple ID on my 6s which I’ll be using while my 8 plus is getting repaired so I tried to restore the back up from the 8 plus but it wouldn’t let me because of the version of ios or my old iPhone is old so my question is have I lost everything on my 8

plus , when I get my 8 plus back will I be able to restore it and get everything back ?