@pvonhellermannn @DrKathyChandler
Hashtags are passive, you only see posts with that tag which have federated to your server anyway (because someone followed their author, or saw the post shared by someone, or it showed up on a relay that your server uses).
Groups are active, they send all their content to all their members on every server, no matter who they federate with.
If you want to see 100% of content on a particular topic, I'd strongly recommend using groups (https://fedi.tips/how-to-use-groups-on-the-fediverse)