2000+ Device network 1 broadcast domain

I had a conversation last week with a senior engineer at a small ISP 2000+ customers.

My background is in guest networks and it came up that most of the time when a guest connects to our networks the broadcast domain is the guest and their gateway.

He countered with their entire network is 1 broadcast domain.

In the moment I gave a confused look and he just mentioned you'll learn a lot more about broadcast domains preparing for your CCNA. But now this is the thought that keeps me up at night.

I knew what a broadcast domain was but at the same time just a few short months ago I thought everything ISPs do was magic and completely different than what I'm doing.

So redditors that have experience in the ISP world. Is it true that ISP networks are just 1 broadcast domain. If so,

1. How do you mitigate the effects of broadcast traffic on your network performance?

2. I was always under the assumption that 500 devices was too many on a broadcast domain. Researching I came across the number of 1022 with a reference to 802.3, reading the latest revision I could get my hands on this appears to be absent from that document (it is hundreds of pages though). Are carrier devices just this good?

3. But really, are you doing magic?

4. Can a fiber ring network (they have many) even be in the same broadcast domain? I learned routers break up broadcast domains and you're jumping through multiple routers if you're in the middle.

I'm reaching back out to the person who had stated this but of course I need to get his number from someone else, but I'll update if I get a response.