How do I stop my iPhone from autonomously changing settings and connecting to unsecured networks?

you stop your phone from joining networks by going into

settings->Ask to join networks

and set it to ask or off

if you turn off stuff like bluetooth in the control center it will automatic turn on again after 24hours, this is by design

if you wish to fully turn off bluetooth you need to do so in settings->bluetooth

If you feel an unauthorized person/app is remotely using, controlling or monitoring your device, then that is possible only if you have done one or more of the following Don'ts...

1. Don't hand over an iPhone to kids or to a stranger without Enabling Guided Access
2. Don't share Apple IDs
3. Don't Jailbreak
4. Don't share sensitive information pertaining to your device
5. Don't give in to Phishing
6. Don't plug in your device in Airports and Public places through third-party cables and trust the device. Beware of Juice Jacking. (Especially in India)
7. Don't leave your iPhone unlocked and unattended in public places like offices, schools, malls, etc.

If one of the above is true then quickly change the Apple ID Password and Return iPhone settings to their defaults.

If a person has ever had your passcode, they could’ve installed something quietly — so the full erase and setting up as new is the safest option. How to factory reset your iPhone, iPad, or iPod touch

Keep the iPhone updated to the latest iOS always and never Jailbreak. That's it.

iOS / iPadOS devices cannot be hacked or infected with Viruses / Malware / Spyware *** unless you have intentionally downloaded spurious software or unauthorized apps directly from the internet and installed them on your device, and/or have Jailbroken. 

It (Hacking) also depends on how careful you are in sharing sensitive and valuable information pertaining to your iPhone such as Passcode, Password, etc with your friends and family members.

Be judicious when sharing the device's sensitive and valuable information with friends and family members.

**The primary reason for this is Sandboxing. All third-party apps are “sandboxed”, so they are restricted from accessing files stored by other apps or from making changes to the device. Sandboxing is designed to prevent apps from gathering or modifying information stored by other apps.

Security of runtime process in iOS and iPadOS - Apple Support

The sandbox on an iPhone is a security feature that creates a restricted environment for each app to run in isolation from other apps and the operating system. It is a core component of iOS's security architecture and plays a crucial role in making iPhones more secure.

If you doubt the authenticity of the information provided earlier, you have two alternatives:

1. Report the hacking incident to local law enforcement authorities and actively pursue the case.
2. Accept the credibility of the information; it is impervious to hacking. Just as some individuals hold unconventional beliefs, such as a flat Earth or moon landing denial, one has the freedom to believe in anything. The choice ultimately rests with you in this open and free world.

If you feel an unauthorized person/app is remotely using, controlling or monitoring your device, then that is possible only if you have done one or more of the following Don'ts...

1. Don't hand over an iPhone to kids or to a stranger without Enabling Guided Access
2. Don't share Apple IDs
3. Don't Jailbreak
4. Don't share sensitive information pertaining to your device
5. Don't give in to Phishing
6. Don't plug in your device in Airports and Public places through third-party cables and trust the device. Beware of Juice Jacking. (Especially in India)
7. Don't leave your iPhone unlocked and unattended in public places like offices, schools, malls, etc.

If one of the above is true then quickly change the Apple ID Password and Return iPhone settings to their defaults.

since i bought the iphone pro max across multiple ios updates i have had the same issues. my phone settings change from my selection, massive amounts of junk phishing or fishing ( idk wich is proper term) spam. I did not "jailbreak" i wouldnt know how. I do turn off as much as possible and delete or disable as much as possible. i dont use or want icloud, because i can never get the data back from cloud, or its a huge hassle, i dont want or use a.i. or siri. I have had multiple cell providers, no difference. i turn my email off, i change it from push to fetch or manual and it reverts back to push. blue tooth activates on its own, i disable air drop n air play it reactivates, i set my phone to require password everytime, it changes on its own, i set wallet to off and not to store credit cards it changes, if i sign in to cloud it grabs my stuff and backsup on its own against my settings. i changed to iphone because my androids n home net were comprimised playing star treck fleet command cell game hackers i stopped all online gaming, i have 2 old offline games i purchased from apple store. i dont share anything. i dont use facetime ( it is disabled) i dont download music movies, i dont use social media of any kind, no 3rd party apps except for dukdukgo safari search. I have reset my phone soo many times i cant count.

nonsense, why would the choice be there if i cant select the one i want

Here's the choices. What specifically do you want?