Hello, when you're on the video in the browser, try right click it and in the menu choose if it is proposed to "copy the link/adress of the video" and the paste it in your src="***" , but it is not always possible I think .
YouTube propose link to embed.
if you try from such a platform,the share button give you access to "copy link".
Don't forget to check license of the media.
As you wrote your src it's like the file was local storage or something.If you refer to a link you should have something with Https...
You can try that way.
Have a good day.
Oh, ok I also experimented such problem between desktop version and app (but with the whole code not functioning and in the other way , code in the Android app and gives nothing when opening on sololearn/pc).
About importing media in the app, I read some answers like you could use a service like Google drive or other from the cloud services (which allow you to share your content as a link) to put your media on and then gives the link to it in your code. I tried with a picture but it has not worked for me...