SSMS UpdatedDTTM not updating automatically?

I'm new to the world of SQL/SSMS (I'm taking a foundational course on udemy and have picked up a lot but am still a beginner). I've been creating some demo tables for practice and need a bit of clarity on the following: I've created an employee table (see pic below), the table has a CreatedDTTM column and an UpdatedDTTM column... the Created is working fine, but no matter how many times I update a certain cell (e.g. change an employee's LastName), the corresponding UpdatedDTTM still shows as NULL. I was assuming that it would update automatically each time I change an entry, or have I got the wrong end of the stick?

Thanks in advance for your help:

[my practice employee table]