Combine vs RxSwift vs Swift collection chains
map, filter, flatMap, reduce. Swift has them on Array. RxSwift has them on Observable. Combine has them on Publisher. The names line up, the chains look the same, and that surface similarity is exactly what makes mixing the three confusing. They are not the same thing.
Three things separate them: sync vs async, eager vs lazy, and whether values arrive over time. Errors, cancellation, threading, and testing all fall out of those.