Can my camera be used to spy on me?

Unless you have jailbroken your phone and installed apps, which someone else can access, the answer is no.

No. It takes work to jailbreak a phone. Now, if you handed your phone over to someone who has the time and ability to jailbreak a phone, that would be different.

No! If you got a message with a ransom request, it is a scam.

No. To jailbreak a phone it must be unlocked and in the physical possession of someone who knows how to do it for about 15 minutes.

It’s true that someone can send you instructions on how to do it, with links to websites that you have to click on to download stuff. But as long as you don’t do that you are safe. Only download apps from the App Store, never from any website.