TSQL - Intermediate results returned by User Defined Functions (UDF) and used within Inline Table Valued Function (iTVF)
When you`re creating a scalar function you define the return type 

CREATE FUNCTION dbo.get_names_udf (@user_id nvarchar(24))
RETURNS NVARCHAR(500) 
AS
BEGIN
RETURN (SELECT dbo.usernames(@user_id))
END
GO


Which was NVARCHAR(500) in this case and SQL server does not need to check the underlying structure of referenced functions/tables as long as knows the return type.

It will fail to execute tho, if usernames function (in this case) would return something other than NVARCHAR(500), which is expected

On the other hand 

CREATE FUNCTION dbo.get_names_itvf (@user_id nvarchar(24))
RETURNS TABLE 
AS
RETURN
(SELECT dbo.usernames(@user_id) as username)
GO


As specified ,returns table therefore SQL server checks the underlying structure of the table because it needs to know what kind of table is begin returned.
How many columns it has,if any of the columns are without names(in aggregate cases) and inform you about it. You can test this by running the query above without referencing the column name( 'as user name' ) in case username table does not have a specified column name.

Note:
I had to tweak this two functions a little bit, to make them work.

Hope this clarifies what is going under the hood for you