3 of 5 added example code

This says to me a subquery (which always returns a single value) represents an empty rowset as null in that return value, but attempting to select a single column from an empty rowset does not return null. Rather, it kind of says "I have no value, so there is nothing to assign".

I think you are looking at both cases incorrectly:

  1. A subquery does not return a result set. It returns a scalar value. When no scalar value exists, it returns NULL for precisely the reason you thought: it means "unknown". Think of a subquery as being a replacement for a scalar UDF. It will always return a single something (T-SQL does not allow for a return type of void). So, what you have is effectively: select @testB = dbo.Function(6); assuming that "dbo.Function" is defined as:

    CREATE FUNCTION dbo.Function (@Input INT)
    RETURNS INT
    AS
    BEGIN
      DECLARE @Temp INT;
      SELECT @Temp = TestTable.[id]
      FROM   (VALUES (5)) TestTable([id])
      WHERE  TestTable.[id] = @Input;
    
      RETURN @Temp;
    END;
    
  2. I wouldn't think of an empty result set in terms of "I have no value, so there is nothing to assign". Instead, think of it in terms of: an assignment will be performed for every row returned, and no rows returned means that there is no action to take. It might be a subtle difference, but the focus really needs to be on "rows = actions" more so than "returned value is something that can be assigned".

    This distinction is not so subtle when:

    1. You are executing a T-SQL scalar UDF in a query and passing in a column. Assuming we are not dealing with Scalar UDF inlining (starting in SQL Server 2019), then scalar UDFs are executed per each row, hence the performance issues associated with them and the impetus for the new feature in SQL Server 2019 to inline UDFs (if they meet the requirements).
    2. You are setting a value like you are showing for select @testA = id from .... The assignment will happen per each and every row returned. And the final value of the variable will be the last row returned. That can be predictable if you use an ORDER BY, or you can live life on the edge and go for unpredictable results by not using an ORDER BY. For example:

      DECLARE @Temp INT;
      
      SELECT @Temp = tab.col
      FROM   (VALUES (1), (3), (4), (2)) tab(col);
      
      SELECT @Temp AS [NotOrdered];
      -- 2
      
      
      SELECT @Temp = tab.col
      FROM   (VALUES (1), (3), (4), (2)) tab(col)
      ORDER BY tab.col;
      
      SELECT @Temp AS [Ordered];
      -- 4