Yes, it is possible to customize the default login screen of Windows 10 without using third-party software. However, the specific modifications you mentioned, such as moving the login fields or changing their size, are not natively supported by Windows 10.
Windows 10 does provide some built-in options for customizing the login screen, such as changing the background image. To do this, you can go to the "Settings" app, then navigate to "Personalization" and select "Lock screen". From there, you can choose a different background image for the login screen.
If you want to make more advanced modifications to the login screen, like moving or resizing the login fields, you will need to use third-party software or create a custom solution. There are various software tools available that can help you achieve this, but they would fall under the category of third-party software.
Keep in mind that modifying system files or settings, including the login screen, can be risky and may lead to instability or unintended consequences. It is always recommended to create a backup or restore point before making any changes to system files or settings.