A little help asking questions or doubts on subject

Hi I used the khanacademy website in android but what I do not know is like I have a few doubts on biology and physics but when I enter the physics library I do not see the ask doubts section or column or button anywhere also when I click on the video it just shows the video as a popup with YouTube options and the tests are also like that so may I please know where and how to asks doubts

Steps from biology classroom will be appreciated 

- Ameershahul29