Button
The Button block is the entry point for button and select menu interactions. This block gets triggered when a user clicks a button or picks an option in a select menu that was created by your bot. In the flow of a select menu the block is called Select Menu.
When a user picks an option in a select menu, the value of the option is available as {{interaction.value}}. If the select menu allows picking more than one option, all picked values are available as {{interaction.values}}. In a user, role, mentionable or channel select these are the IDs of the picked users, roles or channels, see Select Menus.
This is typically used in conjunction with interactive components in your messages.
Here user is whoever clicked. If the message was sent by a command, origin.user is the person who ran it. See Who is user in a sub-flow?.