VMSS, VMSS Flex, what are they? And what’s the differences?

Hello,

I have a situation where I'm planning to use VMSS and attached a managed disk. Could someone help me with below queries:

1. If I'm attaching a managed disk so would it be attached automatically to all the scaling out VM's?

      a. If yes, then hope my data will be persistent on this disk

      b. If No, then will it be attaching new data disks to all the scaling out VM's?

               (*) If Yes, then in case of VM's scaling in, how will that data be persistent? 

               (!) If No, then how can I provide a persistent storage of all scaling out VM's

I might be missing some concept so please share your knowledge to bridge the gap. Thanks in advance !