A semi serious answer. I don't know, and you shouldn't be able to find out much.
I used to work for an equivalent charting company in a different medium.
They'll probably collect sales info from most bricks and mortar stores, they won't get 100% coverage most likely, and then will use statistical tricks to round up sales to represent the whole market. If you want to buy just one track, the mighty Amazon I guess. I'd be amazed if such a big player didn't contribute to the chart figures. The singles chart now includes streaming too. I'd guess from Amazon Prime music, and Spotify amongst others. I'd assume one "stream" doesn't count for the same amount as one purchase, but I don't know the ratio.
You want that song at number 1 when he's here next week? Almost certainly too late. The new weekly chart that Radio 1 uses is broadcast/released Friday afternoon. Though actually, he's here on the 13th right? That's next Friday. Okay, log into Spotify 10 times and play it on repeat all day every day.
I'd imagine that the chart people also do statistical tricks to smooth anomalous data, i.e. data that's too far out of whack. I.e. if everyone else is doing it too, you're probably fine, but if you, say, set up 10 servers all with 20 browsers all logged into Spotify all playing the same random track 24/7, I'd expect the chart company to have something that spots attempts to game the system.
