There are "tall" applications and "wide" applications. Almost all advice you ever read about database design and optimization is for "tall" applications. Basically, it means that your application is only doing one single thing, and everything else is in service of that. Most of the big tech companies you can think of are tall. They have only a handful of really critical, driving concepts in their data model. Facebook really only has people, posts, and ads.

Netflix really only has accounts and shows.

Amazon (the product) really only has sellers, buyers, and products, with maybe a couple more behind the scene for logistics.

The reason for this is because tall applications are easy. Much, much easier than wide applications, which are often called "enterprise". Enterprise software is bad because it's hard. This is where the most unexplored territory is. This is where untold riches lie. The existing players in this space are abysmally bad at it (Oracle, etc.). You will be too, if you enter it with a tall mindset.

Advice like "never user joins" and "design around a single table" makes a lot of sense for tall applications. It's awful, terrible, very bad, no-good advice for wide applications. You see this occasionally when these very tall companies attempt to do literally anything other than their core competency: they fail miserably, because they're staffed with people who hold sacrosanct this kind of advice that does not translate to the vast space of "wide" applications. Just realize that: your advice is for companies doing easy things who are already successful and have run out of low-hanging fruit. Even tall applications that aren't yet victims of their own success do not need to think about butchering their data model in service of performance. Only those who are already vastly successful and are trying to squeeze out the last juices of performance. But those are the people who least need advice. This kind of tall-centered advice, justified with "FAANG is doing it so you should too" and "but what about when you have a billion users?" is poisoning the minds of people who set off to do something more interesting than serve ads to billions of people.