I'm guessing this changed at some point, and either
1. those tutorials are outdated, or,
2. the people writing it for Angular 11 did not keep up?
...and this section gives a bit more perspective. Looks like providers might be an advanced topic...?
But at least, it seems, you can see it's just one of three ways to provide a service. But in some situations, a service can be "provided" using *something else* other than Angular's service class... and I don't know when you'd do that, and how.
https://angular.io/guide/architecture-services#providing-services
But there's also this section: https://angular.io/guide/dependency-injection-providers
which clearly goes: "By configuring providers, you can make services available to the parts of your application that need them."
And now I'm more confused -- this goes over my head. 🙃