The Shulchan Aruch Hilchot Shabbat (siman 243) starts with the laws between a jew and a gentile.
Why does it specifically starts there when it is more likely to discuss the importance of Shabbat at the beginning?
Because Sim 243 deals with procedures preceding Shabbos, like:
דין המשכיר שדה ומרחץ לעכו"ם ובו ב סעיפים:
Laws of renting his property to a Gentile for Shabbos:
One shouldn’t rent out his bathhouse to a non-Jew [before Shabbos] because the bathhouse is known to be his. The non-Jew will do work with it on Shabbat.
After he deals with that he continues with laws that define the relations with the Gentiles that continue into Shabbos.