There's of course some nuance here - for example the original interface was designed to never error, assuming calculation was local in a way. The new implementation of that interface that makes remove calls may now fail - and basically either has to log and return an arbitrary float, or panic, because the interface doesn't allow for propagating errors.
https://gobyexample.com/interfaces
So a struct satisfies an interface if it implements all of the methods of the interface. The example you see in the above link is a monolithic design. Everything only exists in one place and will be deployed together.
So if the functions for circle were getting hit really hard and we wanted to split those out into their own microservice so that we could scale up and down based on traffic, it would look something like this. https://play.golang.org/p/WOp0RL-pVg3
There we have externalCircle which satisfies the interface the same way circle did, but externalCircle makes http calls to an external service. That code won't run because it's missing a thing or two, but it shows the concept.