SWARM VS Nginx .. which is better?

Hello

I read always that ** swarm = high availability** but I found that the tutorials on the net almost use Nginx to provide the “high availability”

I don’t understand why they don’i use swarm directly if it can do the same job as well as Nginx ??

is swarm really able to provide the high availability… or we have to use an other tool with it … please help me to understand