ISPs and telcos receive blocking order. That means almost the entire country loses access.
EDIT: grammar.
Most likely the ISPs drop/block the BGP routes, or just null route traffic for those blocks.
Here in India they just return NXDOMAIN for the name. So it is pretty easy to bypass using 8.8.8.8. Also, the guy who installed the router at my home changed the default DNS to 8.8.8.8. I think ISPs do this on purpose. They don't wanna block sites their customers wanna access. And the government is dumb enough to not understand the difference between a DNS block vs a real block.
Dumb is often good enough for big companies and governments. (See: DRM.)
If you are the sort of person who is able to change your DNS, you are also capable enough to use foreign VPNs etc.
So what happens if you're behind Cloudflare (or similar)? They block all of cloudflare? Cloudflare drops you as a customer?