In the late 2010's, my startup built a messaging app with millions of daily users sending and receiving >1B messages a month, (somewhere north of 300 messages/sec, IIRC) with a trailing 12 month average uptime of our systems of 99.995%

When we were going through due diligence as part of being acquired by a large tech company, I had to answer a lot of questions about our infrastructure and to "prove" that our boring tech stack could actually achieve this.

That "stack" was 3 pods (2 in one AWS region, 1 in another) comprised of 4 Linux boxes and 3 Windows boxes.

In a stack, Linux boxes were 2 monitoring servers and 2 app servers running an OSS SW package, while the Windows boxes were two app servers running our .NET APIs, and a SQL server.

Fail over was handled by SQL cluster at DB level and ELB at the app level.

While the stack and code were boring, a lot of work went into performance, reliability, and observability.

After we were acquired, my new infra budget was based on their own modeling for scale, resulting in a monthly AWS budget that was 10X my previous annual budget.