It's super vague. According to Spotify (on their US website that is) they say:
Spotify pays out the majority (approaching 70%) of ALL of our revenue (advertising and subscription fees) to rights holders: artists, labels, publishers, and performing rights societies...Spotify has direct agreements with record labels, digital distributors, aggregators and publisher collecting societies, to whom we regularly pay royalties, and who then pay recording artists and songwriters according to their specific contractual agreements...we recommend that artists reach out to their distributors to better understand the specific economics that apply to them...In general, however, Spotify pays royalties in relation to an artist's popularity on the service. For example, we will pay out approximately 2% of our gross royalties for an artist whose music represents approximately 2% of what our users stream. A popular song or album can generate far more revenue for an artist over time than it historically would have from upfront unit sales.
It is what it is.
Re: Bandcamp, most checkout plugins/softwares are equipped with the technical/legal/privacy stuff involved with online transactions. It's cool to have a streamlined service like that, but I've yet to hear of it being any band's major source of sales.