how to stop syncing notes between my phone and someome else

You cannot. If you’re sharing an AppleID then you both are accessing the exact same iCloud account and all the exact same data for that iCloud account. It’s really not that you’re sync’ing notes. You are both simply accessing and seeing the same notes folder in the same iCloud account, as would anyone else using your AppleID with iCloud.

The only way to keep any data private or separate with iCloud, FaceTime, iMessage or any other Apple service account is to use separate AppleIDs.

When you share an AppleID, you’re both using the same account as all the system knows is the AppleID loging in, not who is using it.

You’re welcome. Remember you can always use family sharing to share purchased content once you are both o separate AppleIDs. So neither should lose access to, or the ability to update, already purchased apps or content. You can even share iCloud storage if you need more and want to limit the number of paid subscriptions.

Family Sharing - Apple