I believe certifications for digital & streams are just taken purely on chart points, so 1 online purchase = 1 point towards certification. 1 premium stream (ie a paid Spotify/Apple account) = 0.01 points towards cert, and 1 free stream = 0.0016 points towards cert.
In saying this, there may be discrepencies with the ARIA Chart at times because I think (I don't have any proof) for instance, a lot of streaming data is estimated, or at least, goes beyond what ARIA has in their books, like say for the couple of years before streaming was in the chart, and also because ARIA misses a lot of streaming data, and maybe only belatedly corrects themselves going forward, but they don't go back and fix the old data (which they also haven't done for their End Of Decade list I'm pretty sure).
"Africa" by Toto went 7xPlatinum last year, but until a year ago, it wasn't even being counted correctly on the ARIA Chart, so if you saw all of ARIA's weekly stats for it, you'd never think it did that many units, so the certification might be more trustworthy than the chart itself.
I did wonder why the accreditations for the End Of Decade charts were all over the place. It does often seem like there's a catch-up on an international artist's accreditations when they're visiting the country.