At least on my instance, you can click the three dots under the post to open up some expanded options, then click the star to save the post.

On Jerboa, there should be a little bookmark icon under each post.

Yes of course. In the web app there’s a little star icon you can click and on jebora theres a little badge next to the vote buttons

if you’re asking for kbin, I think there is no way at the moment, but I’m sure the developers will add it soon. kbin is a newer aggregator compared to lemmy