You can build ExternalDNS for your platform with make build, you may have to install the necessary dependencies with make dep. The binary will land at build/external-dns.
ExternalDNS's sources of DNS records live in package source. They implement the Source interface that has a single method Endpoints which returns the represented source's objects converted to Endpoints. Endpoints are just a tuple of DNS name and target where target can be an IP or another hostname.
For example, the ServiceSource returns all Services converted to Endpoints where the hostname is the value of the external-dns.alpha.kubernetes.io/hostname annotation and the target is the IP of the load balancer.
This list of endpoints is passed to the Plan which determines the difference between the current DNS records and the desired list of Endpoints.
Once the difference has been figured out the list of intended changes is passed to a Registry which live in the registry package. The registry is a wrapper and access point to DNS provider. Registry implements the ownership concept by marking owned records and filtering out records not owned by ExternalDNS before passing them to DNS provider.
The provider is the adapter to the DNS provider, e.g. Google Cloud DNS. It implements two methods: ApplyChanges to apply a set of changes filtered by Registry and Records to retrieve the current list of records from the DNS provider.
The orchestration between the different components is controlled by the controller.
You can pick which Source and Provider to use at runtime via the --source and --provider flags, respectively.
A typical way to start on, e.g. a CoreDNS provider, would be to add a coredns.go to the providers package and implement the interface methods. Then you would have to register your provider under a name in main.go, e.g. coredns, and would be able to trigger it's functions via setting --provider=coredns.
Note, how your provider doesn't need to know anything about where the DNS records come from, nor does it have to figure out the difference between the current and the desired state, it merely executes the actions calculated by the plan.