How can I tell if someone is watching my iPhone in the background

Griffonpeter wrote:

How can I tell if someone is watching me on my iPhone in the background?

Using the sorts of exploits of the scale and scope and costs you are considering here, you can’t.

If you’re worth those exploits, you will need specialized security help.

For the most of us and the likely scenarios here—those not involving espionage-scale tools—start here:

… Personal Safety User Guide - Apple Support