When I watch Demand shows on the Telus phone app, there is a little symbol on the top left corner of the screen ... a little rectangle with two vertical lines inside of it. When I click on it, I don't notice anything changes. What is this for?
That icon will expand the video to full widescreen. Whether or not it works will depend on the size of the screen on your phone. If it's 16x9 you may not notice any difference. If it's wider like a number of phones (iPhone, S21 etc) that button will zoom the image so that the black bars on either side of it are gone.
This is to go fullscreen. Depending on the aspect ratio and/or screen size of your device, you may already be in fullscreen, which is why you won't see any changes. On an iPhone 12, pressing this button causes the picture to zoom in very slightly to remove the two black bars on either side of the screen. On an iPad mini, it zooms in to remove the black bars on the top and bottom of the screen.