The Singles Charts are calculated daily, and just like in the real world, provide a snapshot of which songs are selling the most on that day. The Singles Charts are simply a reflection of units sold - and unit sales are in turn calculated based on a top-secret formula that weights a large number of factors, including the quality of a song, the quality, level and duration of the promotion, who is doing the Promoting (this can range from individual Blockstars, the entire band and even Record Labels), the Fame level of the Blockstars and band, how competitive the charts are on that day and a number of Hidden Attributes related to Blockstars' personas that can have an impact on chart position and staying power.
There is no one single way that songs will behave when they hit the charts. They can start low, but then even in the absence of promotion they can steadily climb up the charts and get to #1 ("sleeper hits"). Songs from super-famous bands can debut at #1 on the back of strong promotion, but then drop off the charts rapidly if the songs weren't actually very good ("sell-outs"). Songs can rise, then fall, then rise again ("yo-yos"). Totally unknown bands with amazing Social Media skills can come out of nowhere with a chart sensation ("viral hits"). And bands can unexpectedly, for no apparent reason, have a #1 chart topper and then never get close to that again ("one-hit wonders").
The same song, but released by a different band, might behave very differently.