server-less architecture refers to services that allow you to run your code without knowledge of the server it is being run on.
In the case of AWS Lambda, you upload your code and tell it the entry point file and function, and it runs that function, scaling as necessary. You actually have almost no control of how it is run (you can tell it the amount of RAM you need).
It is not that there is no server, it is just that the server is irrelevant and out of your control.
Hope that helps.