Welcome!!!
I use a Polar HR7 heart rate monitor and strap.
Amazon Link
It is solid and works very well.
I think HR straps provide more consistent and reliable data than other methods of recording HR while running.
I used to run with zwift on an iPhone. But Zwift has managed to cram so much information into that small iPhone screen that seeing (it all) required a scanning electron microscope. There are many options for making your screen bigger, and many videos on YouTube for doing so.
Cheers!