If someone logs in on Your iPhone remotely then that is called hacking. Hacking an iPhone is extremely difficult to do (nearly impossible) because the iPhone operates in Apple’s closed environment. What can happen is that your Apple ID and Password can get compromised through a scam or a phishing attempt. In other words, you can get tricked into submitting your Apple ID and password by the perpetrator leading you to log into a fake website. The perpetrator then logs on another device with your Apple ID and Pasword and steals your information that is shared through various cloud based accounts and information that is stored there. The perpetrator can also then view your texts, photos and make changes as s/he pleases. It would be the same things that take place when you share your Apple ID with someone else.
That was a summary and now the answer to your quesiton is to check and see whether there are unknown devices logged into your account. Open Settings ➡️ Apple ID (Your Name at Top) ➡️ Scroll Down and check the list of devices. If there is a device(s) that you do not recognize then tap on it and remove it. Once it is done then change the password of your Apple ID by opening Settings ➡️ Apple ID ➡️ Password & Security ➡️ Change Password.
You should also review the information in this guide published by Apple 👉 If you think your Apple ID has been compromised - Apple Support
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