When I log in to my iPhone it says I logged in on macOS

1. Check your Apple ID under Settings → [Your Name] → Sign-In & Security → Devices.
2. If you don’t see an unfamiliar physical device there, you’re fine — it’s just a server-side label.
3. If you do see a Mac you don’t own, that’s different — then you’d need to change your password and enable 2FA.

I only see sign in notifications when I have truly signed in on a device the Apple has not seen before. When I get a new phone and I sign in on it, I will get that message about having signed in on a new iPhone. I won't see that notification again until I get a new iPhone years later, or for some reason I sign out on my iPhone (such as getting it repaired).

Likewise when I get a new computer I get that notice when I first sign in on it. Unless I sign out of iCloud on my computer and then sign in again, I won't see that message more than once.

Given all that, it isn't clear what exactly you are doing in connection with seeing those messages. A third possibility is that it isn't you doing the signing in but somebody else who has your account information and access to a trusted device (if you use two factor authentication) is signing into your account. If that is the case then your account is compromised and you should read this document if you think your Apple Account has been compromised. --> If you think your Apple Account has been compromised - Apple Support

For iOS 15 or earlier, if you want to see if anyone else has access to your device or accounts, click here --> Checklist 1: Limit device and account access - Apple Support

For iOS 16 or later see how Safety Check on iPhone works to keep you safe. Safety Check for an iPhone with iOS 16 or later - Apple Support

Use the information in this document to check your Apple ID device list to find where you're signed in --> Check your Apple Account device list to find where you’re signed in - Apple Support

Related materials:

Personal Safety User Guide

Personal Safety User Guide - Apple Support

> open the Table of Contents and review the articles

A document with general information about security and your Apple ID --> Security and your Apple Account - Apple Support

Contact Apple for help with Apple ID account security. This page provides country-specific Apple Support contact information ➞ Contact Apple Support - Apple Support

To improve my understanding of the phenomenon, it would be highly beneficial if you could share a full screenshot (Do not crop it). Visual representations carry a greater impact than written descriptions.

Please remember that this is a public forum, so when attaching a screenshot, please avoid including any personal credentials such as IP addresses, card details, email IDs, Apple IDs, IMEI numbers, serial numbers, phone numbers, order IDs, or any identifiable location information if you are sharing a map.

With the limited information that you have provided, It is hard to even guess the problems faced by you. 

You may get faster and better responses to your query by --> Writing an effective Apple Support Commun… - Apple ...

Please re-post with detail so that someone else can help.