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I'm on a Sharkey instance too so I know the answer to a few of these:
- An Antenna is a way to follow tags or keywords, you can create a single antenna with all the tags and keywords you care about and then check the Antenna feed/timeline just like you do the others. You can also create multiple antennas and each of them will be a separate feed/timeline that you can access from the antennas icon.
- It's a custom web page that you can link to from your profile or posts, this was also a thing on tumblr.
- It's basically a bookmark - there's also favorites which is basically the same functionality
- You can add tags to posts, however do not add spaces to your tags - most of the other fediverse software doesn't like that. 2 ways to add hashtags: add the symbol to your post like a twitter post or add it at the end, or use the hashtag button at the button of the create post and then you don't include the hashtag symbol.