1. The number is from an IDC report a few months back that tracks console shipments through June 30th 2019.
2. The report states that XB1 is at 46.9m and PS4 is at 99.8m.
3. The PS4 number is based on shipment data that Sony releases each quarter. In this case Sony later released that the total was 100m, so close enough to IDC's estimate.
4. The XB1 number is based on market data that IDC has access to, such as NPD/GfK and they use this to estimate shipments.
5. Microsoft doesn't release numbers to confirm this, so who knows how accurate it is. It does seem to be in the general ballpark of most analyst estimates.