How to tell if someone has access to my iPhone 12

If you feel an unauthorized person is remotely 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.

No one can remotely monitor your iPhone, unless, see SravanKrA answer. ↑↑↑

1

Some apps may have access to a greater or lesser extent to your location, either when you use the app or all the time. Often when an app uses your location all the time, it is because the app is working correctly. But fortunately, you decide which apps are allowed to access your location, and to what extent. You can see those apps here:

Settings > Privacy > Location Services.

2

Once inside the same menu, check Settings > Privacy > Tracking. These are the apps that have asked for permission to track your use of their app. There you can also choose to disable apps.

3

In Settings > Privacy, you can also check which apps have access to your photos, bluetooth, calendar, etc.

4

If you suspect that someone knows your passcode for your iPhone, you can change it here:

Settings > FaceID & Password.

5

If you suspect that other Apple devices are signed in to your Apple ID, you can see an overview of devices that are signed in to your Apple ID here Settings > Apple ID - look at the bottom. Is there a device you don't know. Delete it and change your Apple ID

How to change your Apple ID password:

Change your Apple ID password - Apple Support