In reply to Elisa.

Patterns are just groups of blocks, and core blocks in this case. So, the responsiveness is either handled by core/Gutenberg or the stylesheet of whatever theme the user has installed. The responsive handling could vary wildly between different themes. It really just depends.

But, you really can't attach CSS to a pattern, only to the blocks themselves.

I would recommend building patterns when using Twenty Twenty-One, which gives everyone a common starting point. There are some definite limitations on that in comparison to building along with a custom theme.