I would not be surprised if the percentage of Apple's code that is in Swift is more or less the same percentage of Google's code using Golang (with Golang having a slight advantage maturity wise).
That could be true, but what it's being used for is perhaps more important than the percentage. Not to underestimate the importance of a calculator, but it is known that Google uses Go in:
- dl.google.com
- Kubernetes
- Youtube's MySQL scaling infrastructure (Vitess)
- Google's mobile data compression proxy (Flywheel)
Of course, Go's first stable release was in 2012, so Swift still has some time to catch up ;).