When phone starts to use adoptable storage? I've recently set 50% of SD card as adoptable, and everything went fine partition is showed in settings, but phone doesn't use it I have installed some apps and they all went in internal partition not adopted one.
When will phone start to use adopted memory, when I run out of internal?
An also can I use adopted storage to save files for example from chrome since it only uses internal memory it's imposible to download on external?
There isn't any rule which says move apps to SD under so and so condition. 
Apps don't move on their own into the ext.SD card (which is now a part of internal storage, since you adopted it). You need to migrate them as described in DiAlex'sanswer here. Also note that some apps ( poorly designed ) after being updated may be 
restored in device storage You will again need to migrate them)
You can move app data or store anything (media, files)  if the app can access it ( I am not sure about Chrome as I don't use adoptable storage) 
For a better understanding of how memory is treated see Mathew's answer here