Hi there,
you cannot edit content in the Customizer it is only for setting global theme styles. Those styles will cascade down to all content on your site where applicable. So what Fernando is showing is the Customizer method of setting a particular typography across your entire site. So thats global styling.
in the Editor some Blocks, provide styling controls, but the core Blocks DO NOT offer mobile responsive controls. For that you would need to use a GenerateBlocks Headline block instead of the core Text and Heading blocks.
See here on how to swap the block and make those responsive style changes:
https://www.screencast.com/t/mW1MwsHy