Remotes
A remote in Home Assistant: a Harmony hub, an Android TV, an Apple TV, a Roku and the like.
On the tile: the remote, amber while it is on with the running activity's name, crossed out while off.
Tap or hold opens the card. On / off under On tap switches instead.
The card
The power key at the top. One row per activity for a hub that has them: "Watch TV", "Listen to music". For a television remote, a ring of four arrows around OK, with Back, Home, play or pause and the volume where the remote has them.
One key on a tile
For a single key on its own tile, set On tap to Perform action and choose Send command. The list of commands comes from Home Assistant's own integration for Android TV, Apple TV, Roku, Sky, DirecTV, LG, Panasonic, Vizio, Xbox and others; for a Harmony, Broadlink or Samsung remote you type the command.