Could someone explain to me how hashtags work here on the #fediverse

If I'm in this instance and searched for #anime, I see 140 posts in the past 2 days.

When @greenhairedfae searched #anime, they only see 65 posts in the past 2 days.

How does this work exactly with adding in posts from other instances? Is it worth being on certain instances if you're interested in specific topics even more? Are there ways to follow hashtags so they give you even more content to see?

I think I just need an explainer!