My favourite thing is themy smartphone, because I can do most of my needworks or tasks with it. My favourite thing is my smartphone, because I can do most of my works or tasks with it.

I read, listen to music, work and study withon my phone. I read, listen to music, work and study on my phone.

I feel like "on" works better in this case.

Yes,It's true that I spend a lot of time at the screenon it , but I can't reduce this time.stop. It's true that I spend a lot of time on it , but I can't stop.

I changed "the screen" to it because you've already established what you're talking about (your phone) The "yes" at the beginning isn't necessary.