I don’t have deeply technical answers to this, but all I have you can understand by reading the Design Concepts documentation Get started with Dgraph
Overall Dgraph proposes to be, Distributed, Transactional, Rebalanced and Highly Available.
When any instance goes down, it is replaced by another in the line*. Be it Zero or Alpha. All Alphas when connecting to the main Zero, know of the existence of the other Zeros and how to find them.
They share information among themselves. To ensure availability. Who is aware of the Zeros are the Alphas. They know where they are and who is part of Quorum.
Dgraph is distributed. That is, all transactions are sent to the final destination in an internally distributed manner. Nothing will need to be interrupted AFAIK.