Hi @MR BILL ,
> the service is normally NT Service
This is default service account, we can change the service account from SQL server configure manager. Suggestion you read the MS document Configure Windows Service Accounts and Permissions to get more information about the default configuration of services of SQL Server, and configuration options for SQL Server services that you can set during and after SQL Server installation.
> where the logged in user or service does not have permissions to the server/database.
SQL server service account is different from SQL server login account.
SQL server service account is that account that SQL service running under . You can change it to Local system that is a very high-privileged built-in account. Or change it to another account has the permission to SQL server service.
SQL server login account is the identity of the person or process that is connecting to an instance of SQL Server. A login is a security principal, or an entity that can be authenticated by a secure system. Users need a login to connect to SQL Server. You can create a login based on a Windows principal (such as a domain user or a Windows domain group) or you can create a login that is not based on a Windows principal (such as an SQL Server login). Suggest you read MS document to better understand SQL login account.
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