Do /tellraw selectors refer to the reader or the executing entity/block?

I have an arbitrary Command Block.

Using minecraftjson.com I set it up to /tellraw everyone: "[ the selector @p ] is a new player on the server. Greet them!".

/tellraw @a ["",{"selector":"@p","color":"green"},{"text":" is a new player on the server. Greet them!","color":"green"}]

When they read the text in the chat, what will they see? Will they read that they are a new player on the server (being them the player closest to themselves) or that the player closest to the Command Block is a new player on the server?

In other words, are selector locations and count in /tellraw commands relative to who reads or who executes?

The selector of /tellraw is only the target for the message, it does not change the executor (like /execute does). So the selector is from the executor's point of view.

The last example here (archive) is pretty much exactly what you want and it says that all players will get the name of the player closest to the executor into chat.