Let's use voting on comments as an example:

When a Reddit user votes on a comment, the user's client sends Reddit a message containing the information "User X upvoted Comment Y." That information gets stored in Reddit's database and is used to do things like increment the total number of upvotes (which everybody sees) and color the upvote arrow button orange (which only User X sees). Reddit knows the identities of everybody who upvoted the comment, but the public is only allowed to see the total sum.

When a Lemmy user votes on a comment, the user's client sends their home Lemmy instance a message containing the information "User X upvoted Comment Y," which that instance then forwards to the rest of the Fediverse so that all instances can get updated with the new information. Anybody who subscribes to the protocol (i.e., registers an instance) can read the message. The user's Lemmy client may only expose the total sum of upvotes in its interface, but the whole list of users who upvoted the comment is available to the public as long as they have the right software to read it.