How can I protect my iPhone 15 Pro Max from unauthorized access?

The titlle question pretty much sums it up. Many yrs ago, he spent enormous sums on any kind everything he could in order to be able to get into my phone(android back then) and I believe he was able to tunnel into my devices because we’d share things about the kids thru Google. He still has an android device and I’ve got an iPhone 15 Pro Max. But somehow he knows things that are impossible bc never mentioned outside of my house or it’s in my iPhone and lately my phone started having odd issues. I recognize the signs. What’s the best free or inexpensive spyware/malware app for iPhone 15 Pro Max? I’m afraid to just download anything.

[Re-Titled by Moderator]