If I turn on, I cloud in Apple, will someone else be able to see it?

I understand that you would like to know whether someone can access your iCloud account from Your iPhone. The answer is YES. If you hand your iPhone to someone and that person either has the passcode to your iPhone or you unlock it for that person, then that person will be able to access your iCloud information from Your iPhone.

Which type of iCloud data are you concerned about that the other person may be able to access? I ask because some data access on iCloud will require entering your Apple ID password as well.

Axel F.