Host candidates are all candidates "harvested" from your network interfaces - virtual or physical. So a 10.x.x.x address or 172.x.x.x appearing that you don't know could be a virtual machine network adapter or a docker network. Other ip addresses could stem from VPNs or other network services (Hamachi, ...)
Server relexive is the ip address a coturn server sees when you connect to it. If 73.157.8.80 is not your public ip, there indeed is a problem. Are you sitting in front of a proxy maybe? What does https://ifconfig.me show as IP?
Relayed candidates are basically the ip addresses of the Coturn servers that offer TURN traffic proxying,
And yes basically all our self-hosted coturn servers get DDoS hammered as soon as we publish the ip.
Global is a 3rd party service that cannot be ddosed.