One side column fixed

Dear all, I’m trying to have one side of a two-columns stripe fixed when the user scroll passed an anchor. I’d like that to be responsive in width also.
Basically, I’d like to reproduce this design :
https://www.wix.com/corvid/forum/tips-tutorials-examples/export-data-collection-to-xls-excel-file-using-code

When scrolling down, the right part stays in place while the other scrolls.

Many thanks !
Baptiste