How can I create a nested json from database?

You can build a wrapper class which maps to your wanted json structure, fill it and return the wrapper instead of the entity from your controller. Your wrapper would look something like this:

public class Header
{
    public int plVersion { get; set; }
    public int mID { get; set; }
    public int sID { get; set; }
}

public class AID
{
    public object orID { get; set; }
}

public class Position
{
    public double x { get; set; }
    public double y { get; set; }
}

public class MContainer
{
    public AID aID { get; set; }
    public Position Position { get; set; }
}

public class RootObject
{
    public int ID { get; set; }
    public Header header { get; set; }
    public MContainer mContainer { get; set; }
}

To be able to easily create matching class structure to a given json you can use a tool like json2csharp.

Mapping the data and returning it:

[Route("api/test/{id}")]
public test Get(int id)
{
    using (Raw_DataEntities entities = new Raw_DataEntities())
    {
        var entity = entities.tests.FirstOrDefault(e => e.ID == id);
    }

    var root = new RootObject();
    // Filling the object
    root.ID = entity.Id;
    // etc. ...

    return root;
}

To have cleaner action methods, I would suggest to write an extension method for mapping the entity to its wrapper object:

public static class EntityExtensions
{
    public static RootObject ToJsonWrapper(this Entity entity)
    {
        if (entity == null)
            return null;

        var root = new RootObject();
        root.ID = entity.Id;
        // etc ...
    }
}

Then call it in the action like this:

[Route("api/test/{id}")]
public test Get(int id)
{
    using (Raw_DataEntities entities = new Raw_DataEntities())
    {
        return entities.tests.FirstOrDefault(e => e.ID == id).ToJsonWrapper();
    }
}

To obtain the resulting json you asked, you must use these classes:

public class Result
{
    public int ID { get; set; }
    public Header header { get; set; }
    public MContainer mContainer { get; set; }
}
public class Header
{
    public int plVersion { get; set; }
    public int mID { get; set; }
    public int sID { get; set; }
}
public class MContainer
{
    public AID aID { get; set; }
    public Position Position { get; set; }
}
public class AID
{
    public int? orID { get; set; }
}
public class Position
{
    public int x { get; set; }
    public int y { get; set; }
}

And in your controller you must do something like:

return entities.tests.Select(x => new Result {
    //Conversion
}).ToList();