Capes do exist on Java, there is the "official" ones that are made by Mojang and work in a similar way that skins does, the difference here is that you can't use a custom cape in vanilla game, only de official ones (For example, Minecon Capes, Migrator Cape, Translator capes etc). If you dont have a cape, then you can't put anything.
On Java all the info related to the player is hosted in a central server, that's why if you play Java without wifi connection you won't have you skin and cape.
On the other hand, mods can connect the client with 3rt party servers, and can give the player custom cosmetics (for example, optifine capes) but other players won't see that cosmetic if they aren't using the same mod, if they are in vanilla then won't be anything.
For example: If you're using optifine, you won't see Badlion cosmetics
The multiplayer servers can't give players client-side cosmetics. The only way it could be possible if cubecraft create a mod and force everyone to use it for being able to join the server, but there isn't a good reason to do so.
As far I know this is how it works, if someone found a error in the info feel free to correct me. Have a good day!
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