When you requested the free download I think you should have been asked to enter your email address, or perhaps when you first opened the program. Whichever, the answer to your question is that the serial number will be emailed to you if Magix have your email address! If not, it will run for 7 days (in Demo mode) then stop, I think.
To answer your question: yes, it is free but you do need to register.
No email was requested upon download. I am already a customer of MANY Magix products. If there was an email requested I would have checked my email and found the serial number.
The serial numbers are sent when you pay for Magix products.
I am aware of how purchased products go but the free ones have no path to get a serial which is why I posted the question here. Still not sure what to do when no email is requested but the download starts anyway....