Also each time the UI is opened, the widgets are reset at maximum (not the values).
Glad to see you're excited and working hard on learning LV2! Checkout the example plugins: they show how things work.
In the UI, there is a port_event() function. It tells the UI what port changed, and what its current value is. Use this to do widget->setValue( newValue ); and you're done ;)
Eg (from OpenAV's ArtyFX, Bitta plugin)
How can I get the control ports values from the UI perspective?
In answer to your Q, how to "get", you don't: you're told when there's a new value.
HTH, Cheers, -Harry