They would not have traveled there. If Marco Polo made it as far as China (there is some debate but let's just accept that he did for now), he would have been the exception and not the rule. Furthermore, there isn't a great deal of value in what is now modern Mongolia, so there probably would not have been nearly as many people there as there would have been in China.

We know from Ibn Battutah that there were Muslim quarters in medieval Chinese cities, possibly of Turkic origin. They would have been the best example of non-East Asian merchants deep in the Mongolian Empire. It's best to think of global trade in the Middle Ages and before as segmented. Goods passing from middle man to middle man before Europeans bought goods in the Levant, Alexandria, or the Black Sea region.

There are translations of Pegolotti, a trade manual that detail these networks, but I'm afraid much of it remains untranslated from an antiquated Tuscan. You might also look at accounts of Franciscans in Mongolia, which can be found here in an antiquated, but free translation. John of Plano Carpini and William of Rubrick both journeyed and interacted with Mongols: https://archive.org/details/textsandversion01ruysgoog.