Screen dpi is hardcoded in each device description. In the case of android it is obtained using the native framework and
it should always match your device DPI.
UI elements are not scaled by DPI unless you override your device defaults. They're scaled by the physical resolution of your device.
See
https://github.com/koreader/koreader....lua#L371-L383 and
https://github.com/koreader/koreader...ze.lua#L31-L80