The phone just becomes the processing power; essentially an ARM laptop with all of the peripherals external.
I currently using Pixel 9 Pro XL (512GiB) and I imagine it's got more compute power than my ageing 2019 XPS 13.
Conversely, I'm not entirely sure what the use-case is. It couldn't replace my work-laptop with a 20-core CPU and 64GiB of RAM and ARC GPU, running Ubuntu/Gnome that I can also connect to a couple of monitors, keyboard, mouse, speakers, webcam, and more with a single cable via a docking station; and if I was going to carry the peripherals needed to do this with my phone, when on the go, I'd just carry a laptop, like I do now.
Curious to hear what the use-case is for people with these desktop/phone crossovers. If it's to cover the use-case where I haven't brought a laptop with me, forgotten it, didn't bring it for weight or whatever; where am I supposed to find these peripherals to use?
The problem is, the OS is very limiting. The file manager of iOS is extremely dumbed down, even Firefox doesn't work properly on it, and desktop mode with stage manager is just stupid. So yeah, it's not a success so far.
Android though feels more open, so I would give it a try of their desktop mode isn't stupid. Firefox on Android is fully functional at least and that's a great start!
But, more broadly, the problem is that they can do it for their own apps, but for all the third party ones, it's down to app developers whether they want to put in effort to support keyboard & mouse/trackpad properly. And it's a chicken and egg thing there - relatively few users use it currently, so there's little motivation to add support, which deters more potential users.
Funnily enough Windows is the closest to get this right with 2-in-1 tablet/laptops. 2 problems with this: 1- Windows, 2- sub-par HW.