Where does Eureka Server stores the service details to maintain its registry?

Question

I'm new to Spring Cloud and was checking the Eureka tutorials. I have few questions which I couldn't figure out

1. Where does Eureka service registry stores the micro services details? Is it in a in-memory hashmap or any metadata log?
2. During the health check of services, is the connection establishment between the client and the service is UDP/TCP/through web sockets ?
3. How replication of eureka server really works? Does peer awareness provides replication ?