Everything is done by tags here for discoverability, and there's no browsable tag list or pre-genned tag feed; you gotta DIY.
IDK if it's been said in addition to the above, but if you want to try to find out what tags are used for what, there are only two ways (AFAIK), and they're both a bit tedious (in service of a Better Platform tho), so here's the two:
In the new post box, the '#add tags' tag box at the bottom is a live search - start typing a tag (no # required) and it'll give you a list of mostly sorted-by-popularity tags starting with those letters, so you can just start fumbling thru to find a tag you want to use.
To check if that tag is ACTUALLY used at all tho, you need to go to 'Search' on the left side of Cohost, search for that tag, click on it, and see if it has chosts.
If there's more than one variant, here's my trick for figuring out which is the most popular of the tags: go to the next page of a tag, and note that the URL of the page now indicates skipped posts. Increase this number manually, until you see no posts, then slowly decrease until you find the oldest post in that tag. The number in your URL will now generally indicate how many posts that tag had at the time of your search! Boom. Tag numbers.
For reference, 'The Cohost Global Feed' is one of the most popular tags around, with well over 20k posts in it. But don't focus on how many posts a tag has; use this to gauge if the tag you want to use is a tag others are using too, if your goal is categorical discoverability.