ORIGINAL_LOGIN() function returns the name of the original login that connected to the instance of SQL Server and is used to identify original login in all sessions. Even though you will do the security context switch it will return the original login name.

SUSER_SNAME returns the name of user in the current security context.

--connect SQL Using LoginDEMO account
SELECT ORIGINAL_LOGIN() Original_Login_func, SUSER_SNAME() Suser_Name_Login_Func

--Executing query using LoginTest account
execute as login = 'LoginTest'
SELECT ORIGINAL_LOGIN() Original_Login_func, SUSER_SNAME() Suser_Name_Login_Func
revert

--Again executing query be reverting the change
SELECT ORIGINAL_LOGIN() Original_Login_func, SUSER_SNAME() Suser_Name_Login_Func

Check below output image for more information. I connected SQL Server using LoginDemo account.