What actually happens if I turn off iCloud?

Depends which switch you mean. Turning off iCloud for one thing on one device (say Photos or Contacts) stops that device syncing it. What's already in iCloud stays there and your other devices keep it. The catch is Photos: if you had Optimize iPhone Storage on, the full-size originals only live in iCloud, so when you turn it off the phone asks whether to download them or remove them. Pick download, and make sure you have the space. Signing out of iCloud completely asks whether to keep a copy of things like contacts and calendars on the phone. Say yes if you want them there. Neither one deletes your iCloud data, and neither cancels a paid storage plan. That you have to cancel separately if you want to stop paying.