Please describe where and how my info is safe in iCloud

I am 57 year old female who has worked in the IT field and have used computers since 1993. I am baffled the the iCloud concept. Even though I can read what the "product" is and does for me I can't find it anywhere exactly where does my information sits (some server in outer space locked down in 12 key security chains?)??

How is it safe from hackers (yes I know the IOS is safer because is a closed system)? So when I have all my e-info stored in one place how do I know it is not being used and sold by Apple for marketing purposes etc...This is like renting my brain space to someone else...even Apple has its weaknesses in security, Iam sure...SO called me old fashion or just old but I can't swallow this "iCloud " concept (for my own computer detriment I'm sure) unless I understand how safe my info use and how guarded it is. Can anyone in my age group shed some light on this? I am a medical provider and use a cloud based system for my computing but I can't do it with my iphone/macbook/desktop yet....help me. Thanks!