Yep it's another blockchain running in parallel, called the "beacon chain." Anyone can deposit ETH in a particular contract on the main chain, and the beacon chain will read that and credit them ETH on the beacon chain.

The quickest migration is basically to upgrade both clients to talk to each other, and for the old client, instead of choosing blocks with the highest hashpower, choose blocks whose hashes are chosen by the beacon chain. Details here: https://notes.ethereum.org/@vbuterin/B1mUf6DXO

The next step is a more integrated system, described here: https://ethresear.ch/t/executable-beacon-chain/8271