Compared to Google Assistant and Siri, Cortana can do essentially everything that the others can do, even though it doesn’t have the same level of access to iOS and Android devices as native assistants do. But, it’s possible to set Cortana as the default assistant on Android devices.
Last year, it was stated by Microsoft that Cortana had over 150 million users across its ecosystem, including Windows 10, ANdroid, Xbox, and iOS. Cortana can even be extended to third-party hardware devices using an SDK strategy.
Even with Cortana being on Windows PCs, Google Assistant will potentially be coming to all Chrome OS devices (i.e., Chromebooks) in the not-too-distant future. Cortana’s implementation will be at the device maker’s discretion.