Sharing permission vs security

The Security Tab is the NTFS permissions.
Sharing permissions only apply when a user accesses a share via the network.

If the person is not given write permissions at the sharing level, then no matter the NTFS permissions, they will not be able to write to the folder.

Folder/NTFS permissions apply to a local logon or network access. If sharing
permissions conflict with folder/NTFS permissions for a network user, the
most restrictive permission will apply to the user. In other words if a user
has only read access in the sharing permission but full control to the folder/NTFS
permissions. That user will only have to read/list/execute access over the
network for the share contents.

Reference: 1 2

System account in Windows

The system account and the administrator account (Administrators group) have the same file privileges, but they have different functions. The system account is used by the operating system and by services that run under Windows.
By default, the system account is granted full control to all files on an NTFS volume.
The system account's permissions can be removed from a file but it is not recommended.
Reference: here