For reasons that we don't quite understand Apple has hidden the option to set an event to free or busy on macOS. So it seems that all-day events you create are always "free" and other events are "busy" with no way of changing it.
However, there is a "Show As" option on iOS that lets you choose between free and busy. When editing the event you scroll down a bit and can see this option.
Also, keep in mind that "read-only" calendars (e.g. national holidays) use the free-busy status as described in the events themselves so it would depend on the underlying feed.