RoundTripper interface is like
type RoundTripper interface {
RoundTrip(*Request) (*Response, error)
}
and inside net/http/transport.go I can only find an function named roundTrip(which is lowercase), so I wonder where does this struct Transport implement RoundTripper interface?
The function you are looking for is in https://golang.org/src/net/http/roundtrip.go instead of https://golang.org/src/net/http/transport.go.
To see this you can try this playground and its output - https://play.golang.org/p/A97ylJmc43R
The code was in https://golang.org/src/net/http/transport.go, but was later refactored out in https://github.com/golang/go/commit/b12e341616365cafd6f7eab9aaaa29c4155c1a76#diff-e85f2dbad3447c4267e815cc9d014e69.
(Speculation - do not trust it 100%) From the commit description and the issue (https://github.com/golang/go/issues/25506), it seems that the refactor was for using the Fetch API (https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API) in the WebAssembly backend (build flags). Since this would only work for WASM, the code was refactored to split out the two separate implementations.