Digital photo cameras often actually filter out infrared. But even if they wouldn’t it probably would be hard to tell if something is warmer than the surrounding by looking at a photo or video. What you need is a specific device that is calibrated for a specific spectrum of infrared, such as a thermographic camera.

I think there was actually at least one smartphone that had a thermographic camera installed, but that’s a very specific use case for e.g. construction work.

You specifically want an external accessory like FLIR for that, only a rare few industrial range smartphones have heat cameras built in

No.

Pixel 8 Pro has a thermometer. Not sure if it works on water.

Some of the Ulefone Armor phones have IR cameras. Unless you love construction it's probably not the best purchase for the average person.

No but my cheap work phone does have an adapter.

Some do yes we have FLIR on the cat S62

They do on the Yautja planet.