There is a technology released for the PS4 and working on PS5 called Playlink.

https://www.playstation.com/en-gb/accessories/playlink/

This uses the playlink technology to link your smartphones to a Playstation to use as controllers. This only works with supported games.

This is a brilliant idea for multiplayer games, such as boardgames and card games where you don't want the other players to see your cards.

I've tested it with Ticket to Ride and it works really well. You see your train and destination cards on your phone and your moves are shown on the PS4/5.

There are other games that support this such as Uno. But I've not tried this yet. Games like Monopoly would also work really well.

This would also work brilliantly for multiplayer local stategy games and RPG stategy card games. I hope more devs explore this and it doesn't get abandoned.

The only limitation is the need to download the corresponding app for the game from the Apple or Android stores, so it will only work whilst the apps are available and compatible with the mobile OS. So this isn't going to work forever, other then keeping older smartphones with the apps installed specifically for this.

This is sort of an evolution from similar attempts in the past. Closest would be the Wii-U and it control with a screen. And going back further the Gamecube linked to GBAs, but the only game to really utilise that was Final Fantasy Crystal Chronicles. It even back further to the Dreamcast with it VMU memory card with the little LCD display. I've always through a separate controller screen was a great idea, and utilising smartphones is a logical solution now most people have one. Friends can just install the app and join the game.

I wonder if anything uses Smartphones on Xbox or PC?