My understanding is that custom license field are saved on customer's machine once downloaded along with activation info from LimeLM servers.
Correct.
How/where they are then stored in customer's machine?
They are stored as part of the activation data, and thus are cryptographically signed.
I am interested if they are easily accessible form customer's point of view or or copyable to other machines.
They are copyable to, but unusable on, other machines.