Troubles adjusting for mobile

I am trying to make a website responsive in the gutenberg editor with FSE, but even using measures that are not pixels, like em often does not completely satisfy every element. Like for example having two paragraphs next to each other. I can technically enable “wrap to multiple lines” but there is no conditioning, so it just decides to do it in the desktop version as well. Or for example having a wide looping video behind some text and on mobile it stays stretched out and blurry and I cannot resize it with em or %. What do I do in cases like these?

 

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