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?

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.