What does the line under the Wi-Fi and 5G icon on my iPhone screen mean?

It's a reminder that you can swipe down to open the control center.

You can deactivate Control Center, go to -> Settings -> FaceID & Passcode -> Scroll down and deactivate Control Center and it will not appear.

Keep in mind if you have Face ID enabled and look at the phone, it will still appear.

I’m pretty sure someone has got into my phone.

To keep your Apple ID account secure, see the following Apple articles: 

If you think your Apple ID has been compromised

If you think your Apple ID has been compromised - Apple Support

Recognize and avoid phishing messages, phony support calls, and other scams

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

Use the built-in privacy and security protections of iPhone

Use the built-in privacy and security protections of iPhone – Apple Support (UK)

See who has access to your iPhone or iPad

See who has access to your iPhone or iPad – Apple Support (UK)

Delete suspicious content from your devices

Delete suspicious content from your devices – Apple Support (UK)

Check your Apple ID device list to see where you're signed in

Check your Apple ID device list to find where you're signed in - Apple Support

• if you see devices that don't belong to you, delete from your account.

Personal Safety User Guide -> Personal Safety User Guide – Apple Support (UK)

Manage and use your Apple ID -> Manage and use your Apple ID – Apple Support (UK)

(if you haven't already done this) Two-factor authentication for Apple ID -> Two-factor authentication for Apple ID - Apple Support

If other people have access to your iPhone or other Apple device, change the device passcode and never share it with others.

Change your Passcode -> Use a passcode with your iPhone, iPad, or iPod touch - Apple Support

Change your Apple ID password -> Change your Apple ID password - Apple Support

• Create a strong password for your Apple ID, It is best if your password consists of eight or more characters and include upper and lowercase letters, and at least one number. You can also add extra characters and punctuation marks to make your password even stronger. 
• Never provide your password, passcode, security questions, verification codes, recovery key or any other account security details to anyone else. Apple will never ask you for this information.

Then see this Apple article: What to do after you change your Apple ID or password

What to do after you change your Apple ID or password - Apple Support

Never jailbroken your iPhone.

No one has got into your phone unless you let them use it for an hour or so and also provided them with your Apple ID and Password.