How Canada's Internet went down?

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On 10th July 2022, Canada's internet was DOWN. People were flocking to coffee shops/libraries for internet. I could not get in touch with my friends. How can complete country's internet go down like a house of cards ? Let's dissect it through our technical lenses.

Before we go to the actual technical issue, let's focus on how do we or anyone in the world gets internet ? You get it through some company or what you call ISP - Internet service providers.
They are a lot of telecom companies providing internet in India including Airtel, Jio, Vodafone, ACT etc. But, in Canada, there are only 3 ISPs which control 90% of Canada's traffic. These are: Rogers, Bell Canada and Telus Corp.
The issue was with Rogers which alone provides internet to 11 million Canadians. They came out with an apology calling it a maintenance issue. But, we techies don't stop at that.
Cloudflare which monitors internet traffic gave a more detailed analysis, saying that the outage was caused due to a glitch in the Border Gateway Protocol.(BGP)
Before we go to BGP, let's first understand what internet is from its textbook definition. Internet is simply a network of networks. So, you are connected to one of the network which is your ISP's network which is then connected to other networks on which you are able to access information. Now how is this ISP connected to other networks ?
That's when Border Gateway Protocol comes in. BGP allows one network to advertise its presence to other networks which together form the internet. Rogers stopped advertising, so no other network could find its presence and hence no one on Rogers Network could connect to anything on the Internet.
This can happen to any website which is not able to communicate its presence outside it's local network.