YoutubeApi v3 - get videoId from activityId

Using Youtube's v3 https://developers.google.com/youtube/v3/docs/activities/list API, is there a way to access videoId?

I can't seem to find a way to do it. Is it even possible?

I tried using youtube's search api, but it costs 100 units, which makes it to costly to use.

Am I missing something here?

Thanks