Is Volume actually Apple Ads Keyword Popularity?

The answer to this question is both yes and no. 

In the US, we directly print the Apple Ads Keyword Popularity, but in other countries where Apple Ads is active, based on our users' requests by default, we show our own volume.

If you are interested in Apple Ads data, however, you can go to the Keyword Research tab under ASO Intelligence and have a look at the chart named Volume Popularity History.