emptying icloud
Help!
I carelessly upgraded to High Sierra 10.13.1 ("carelessly" meaning I didn't read about changes). Computer in use (MacBook Pro, early 2015) began making references to iCloud, but I ignored them, thinking if apps were backing up to iCloud it couldn't hurt anything.
Then I got rid of old i4 phone. Presumed newer iPhone telephone contact list would be empty and would need populating with contents from my old i4. Surprised to see new phone had all of my contact information from Yahoo email account! I DON'T WANT THAT!. However, if I delete a contact from my phone, somehow the iCloud system also deletes it from my Yahoo account. I DON'T WANT THAT!
I don't want iPhone (SE) or apps going online or using anything but my phone number unless I tell them to, and I don't want my computer/apps communicating with iCloud account unless I tell them to. (I want to keep information on the MacBook separate from information on the iPhone.) Also, I want my files and information residing in storage on my devices only, unless I make a conscious decision to send them to iCloud.
How do I stop apps from sending information to iCloud, without losing access to what they've already sent there (since, I presume, that information is not residing on my devices). In other words, I want my information back on my devices before I terminate connection with iCloud.
Am I even understanding accurately how the iCloud system works? I once was leading edge (Imsai 8080 with 2 huge Altair disc drives & dumb terminal), but now I'm 71 and no longer even close to savvy! Someone straighten me out. THANK YOU!