I am afraid you didn't describe this correctly.
"pietr" is not called a shadow account. It is the name of the user profile.
"pietr" is also not a local account. If it is already linked to Microsoft account, then it is no longer a local account. You either log in as a Microsoft account user, or a local account user, it cannot be both at the same time for the same user profile.
A user profile is given whenever you set up a user in Windows.
Think of it like when you go to a hotel, you are assigned a room number, and an actual room. The room number is the username, and the actual room is the user profile, which houses your items, like desktop, pictures, documents folders, etc.
You can set up a local user, the local user's username by default will be also the user profile name, what you see in C:\users. For local user, you define a password for it.
If you set it up as a Microsoft account, then the user profile name by default will be the first 5 letters of your Microsoft account email address. And the password is your Microsoft account password.