I just learned how to collapse threads in Jerboa.
You can long tap on the username of the thread you want to hide. It was not the most intuitive thing for me but I found it accidentally and maybe I can save you from the searching!
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Copied from another explanation: >use your search feature on your main instance and enter !{community_name}@{instance_url} then you can subscribe
So for example if you want to sub to the memes community in lemmy.ml then go to Search, type in "!memes@lemmy.ml", under Search change the dropdown from Communities to All (important step), then you should see the community which you can subscribe and post to.
Edit: added image for better understanding.