If your screen, after compensation for DPI, runs at what our graphics designers have deemed large enough for the full ui, you are in tablet mode (which is similar to the desktop mode). If your machine doesn’t hit that spec, you run in phone mode.
The resolution, post DPI compensation is around 1024x700.
What most people around here call portrait mode is really phone mode, and waht they call landscape mode is desktop/tablet mode.
The phone mode does not really have browsers… you can get to them, but it’s not easy. The phone UI is meant to be a reduced functionality UI, for quick actions based around mobile usage, and not as a primary Roon experience.
It is on the roadmap to make the phone experience first class, but it involves a lot more than just a UI change.