Ask HN: How Does Twilio, Stripe, Cloudflare and YouTube Work?

As CRUD web developer I don't have in detail answer to how things work outside of browser/databases/network.

If someone asks me what happens after you type URL in the browser I can talk about DNS, NAT, TCP, Load Balancer, Https Handshake, Caching, CDN, Ports (destination/source) and so on.

Yet I don't know how would I build twilio. Or how does it work. How do you make a call or send sms. Do you have to get contract with mobile network company and have them provide you an api??

Same with stripe. How do they connect with banks?

Same with Youtube/twitch how does streaming work? what techs are used to broadcast stream to millions. I can upload videos on CDN/S3 and link it. But that's not it. What If i want to build tutoring service where 30 students can watch stream of a tutor.

Etc

Please share your knowledge in as much as depth as possible.