How can I add new devices to my appleId?

If you set up your new device with your Apple ID, it will automatically be added to the list of devices.

Also check this discussion:

HOW DO YOU REGISTER A NEW APPLE DEVICE? - Apple Community

Not all devices will show up in this list, for example the older AirPod models.

Go to this document -->

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

See the section:

Add a device to the list

To add a device to the list, you need to sign in with your Apple ID. After you sign in on a device with your Apple ID, that device appears in the list. Here's where you need to sign in:

For iOS or iPadOS, sign in to iCloud, iMessage, FaceTime, iTunes & App Stores, or Game Center on the device. You can also see your paired Apple Watch.

For macOS or Windows 10 and later, sign in to iCloud on the device. 

For Apple TV HD and later, sign in to the iTunes and App Store on the device.

For Apple TV (3rd generation) and earlier, sign in to the iTunes Store on the device.

Your AirPods will appear in the Find My app, but they won't appear in your Apple ID device list. You can remove your AirPods from your account using Find My. Devices running iOS 7 or earlier or OS X Mavericks or earlier won't appear in your device list.