Do web developers generally create their own audio subsystems or use platform native APIs directly? No, those aspects are handled by the browser which does the

Do web developers generally create their own audio subsystems or use platform native APIs directly? No, those aspects are handled by the browser which does the heavy lifting for them, allowing for a write-once-run-anywhere{-that's-supported} experience. The cross-platform toolkits (like Qt, JUCE, etc) do the same thing; they talk to the platform APIs on your behalf. What kind of expertise or team structure you require to build your application for various platforms really depends on what level of the stack you want to author against.