It's a chain of providers.
Generally speaking, you have your wholesaler who sells to individuals or to other iptv resellers (eg. bulk discount of monthly subs).
A wholesaler will generally pay monthly to other source stream providers from a variety of sources to create a full catalogue of channels to provide to individuals and resellers. The wholesaler usually does the EPG management also - basically sources EPG from a variety of data sources.
So UK channels might come from one source, Canadian channels from other, some channels are on this cable provider, others on another - could be different sources. These sources are people who buy cable/satelite set top boxes and subscriptions to then stream them to the wholesalers.
A source might sell to 100+ wholesalers, and then those wholesalers will have 1000+ of subscribers each.
Generally, don't subscribe with a re-seller...it's your best chance that you'll have to find a new subscription. You'll get the best support at wholesale and they hang around longer, although being bigger makes them targets for raids or even to close up shop and take all future subscription money.