Actually, I use my iPhone with a USB-C/HDMI cable, the Remote Desktop client and a Bluetooth keyboard when traveling. Some apps will let you use an additional display just fine.
OK so I've now tried this with a new USB-C iphone.
Yeah it's painful to use! You can set up a mouse, and use a physical keyboard for input, but it doesn't attempt to do any more than mirror the screen onto the external device by default.
Huawei's desktop mode was limited, but I think you're right - you can say the iphone has good device compatibility, but there's not a good way to use it docked. Not that the android ones were 'good', but they made an attempt!
Which is quite frankly weird, given that the iPad has fairly robust mouse/keyboard support at this point, and at least some nods towards window management