Video/image in background of a page of my Ionic app

Hi all,
I’m finding it difficult to put video in the background of my app Ionic.
I believe that I lack the knowledge for this,because I have a video that is well shown on Galaxy s7 edge Samsung but bad on Huawei p20 mate lite. Im using the tag video>" .The difference between the two phone is the size of the screen.But how i must do for put a video in background of the page of Ionic so the video is shown well in all two phones???
Also if i put an image in background i have the same problem…
Thank you all!!