For apps, Artemis exists, but only works on artemis.camp until the API is official, but you can grab it and check it out. Pretty sweet already.
Upvote means "I like what you said and want you to know it".
Boost means "I like what you said and want others to see it too!". Think of it like a re-tweet or whatever it's called on Mastodon (re-toot?).
ETA: I believe the dev of Artemis is also planning on open sourcing their KBIN<->lemmy translation layer once everything is confirmed to make it easier for others to add support to their apps too
Vuipes