"The Fediverse" is more than just Mastodon. The Fediverse is many platforms that talk to each other using a common technical standard called ActivityPub.
If you're using one Fediverse platform, you can also interact with people on other Fediverse platforms, because ActivityPub connects them together.
Shares, follows, likes, comments... these things federate across the Fediverse from one platform to another. This happens so seamlessly that you may not even realise it is happening!
Try it yourself below:
@amogurumi is on PixelFed, a photo platform
@craftykat is on PeerTube, a video platform
If you access these from Mastodon, they look just like Mastodon accounts, and you interact with them in the same way too.
However, if you click on their profile avatar images you will see them on their home platforms.