How do eSims work (affordability not technically)

I understand how an eSIM works, my questions is as follows. How does a travel eSIM country get the ridiculously cheap rates for travel? I've got an upcoming trip to Europe, and on a whim I wanted to check to see what rates for Canada would look like.

Universally, the price / gigabyte is cheaper for one of these travel eSIM country than what I can get from my provider (or competition).

If a company has an agreement with a local provider, how does it determine which provider to connect to to get that rate?