Does ActivityPub send those to other instances, or does ActivityPub only send the original post and the rest (upvotes, downvotes, replies) are stored only on the original server where the post was made?
Does ActivityPub send those to other instances, or does ActivityPub only send the original post and the rest (upvotes, downvotes, replies) are stored only on the original server where the post was made?
Basically. For example, ActivityPub objects such as events or locations aren’t supported by many platforms (though they do exist).
Exact implementations differ per platform. Mastodon doesn’t have a like button, but it does have a favourite button, which is translated into a like when the activity federates. Downvotes are implemented as dislikes (an Activity Streams 2.0 feature, not part of the ActivityPub spec itself) but Mastodon just ignore those.
Furthermore, there are tons of extra JSON fields and extensions that allow servers of a particular type to talk to each other better. For example, take the JSON returned when I query for details on your user account:
curl -LH 'Accept: application/ld+json; profile="w3.org/ns/activitystreams"' https://lemmy.world/u/Tehhund | jq { "@context": [ "https://www.w3.org/ns/activitystreams", "https://w3id.org/security/v1", { "lemmy": "https://join-lemmy.org/ns#", "litepub": "http://litepub.social/ns#", "pt": "https://joinpeertube.org/ns#", "sc": "http://schema.org/", "ChatMessage": "litepub:ChatMessage", "commentsEnabled": "pt:commentsEnabled", "sensitive": "as:sensitive", "matrixUserId": "lemmy:matrixUserId", "postingRestrictedToMods": "lemmy:postingRestrictedToMods", "removeData": "lemmy:removeData", "stickied": "lemmy:stickied", "moderators": { "@type": "@id", "@id": "lemmy:moderators" }, "expires": "as:endTime", "distinguished": "lemmy:distinguished", "language": "sc:inLanguage", "identifier": "sc:identifier" } ], "type": "Person", "id": "https://lemmy.world/u/Tehhund", "preferredUsername": "Tehhund", "inbox": "https://lemmy.world/u/Tehhund/inbox", "outbox": "https://lemmy.world/u/Tehhund/outbox", "publicKey": { "id": "https://lemmy.world/u/Tehhund#main-key", "owner": "https://lemmy.world/u/Tehhund", "publicKeyPem": "-----BEGIN PUBLIC KEY-----\nMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAr8QYBRNqyM3A8JHL+rWD\nN22EJDEBd+1D8hzbOnevWnmalBhbp94MY5xyTCOfGIxYo1tZs5BeuM79JRT7eFV6\nefSPZclwri4XOmizgMY2VVRw2zH3zVXmKjbIn84JaNIUez5z5NAtqgzPr+UDxWIZ\n2lH0kJuZ2YBBvH3Bk1xsJznQ3olnh0hGD9+wU10fTSI4d/razTO+4btOMV5yQYry\noZ3RWD4Zq9nhKw5s4Sb5QPQ0NNHnPsnsZPip5FfN67XOQn/d/H2TzBAdKUtEIVBH\nDivI3FWPWmCbdaz3LImS5FpKNoJvoh7Dwlfh2eIE7mkZ9FH64DNw6cd6A2fSOm1w\nXQIDAQAB\n-----END PUBLIC KEY-----\n" }, "endpoints": { "sharedInbox": "https://lemmy.world/inbox" }, "published": "2023-06-11T19:07:49.583473+00:00" }Notice the special fields for PeerTube, LitePub, Matrix in the context object: these are additional fields to provide optional metadata for compatibility, in case they’re necessary. In your case (and in most cases to be honest), they’re not used.
ActivityPub has a relatively simple core architecture with lots of flexibility. You can ignore most of that flexibility to get an extremely simple client, or you can go through every server and find all the rich content they provide to build the mother of all social media apps.