what’s the benefits of using Lemmy compared to the others? I found this picture on Google.

They all serve different purposes.

For example Mastodon is for "microblogging" or basically what you do on twitter. Everyone yelling out a few SMS messages worth of stuff and others can reply to it.

Lemmy is a "news aggregator" or basically what Reddit does. People post on a topic and others create threads of discussion on the topic.

These two services arent serving the same purposes. What they share is an underlying protocol for doing federated social networking.

I still don't understand how I this services communicate each other. Ok, they use the same protocol, but practically how can I, on Lemmy, access content from PeerTube or Mastodon?