If you follow or search for a hashtag, it can only show posts that your server has noticed anyway. (If you're interested, there's a complete list of what servers notice here: https://fedi.tips/which-posts-and-accounts-can-i-see-from-my-server/).
In this sense, hashtags are "passive", they are basically filtering what your server can see anyway.
If you want a more active way to see what is going on, try following groups. They actively push all their content to everyone, more info: https://fedi.tips/how-to-use-groups-on-the-fediverse/
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