would use any cash, be it Euro or RMB, in their purchases.  That would be too risky and stupid.  Most likely they used plastics.  However, there are many ways they can use plastics.  It'd be easiest and cheapest to transact if they just used plastics, e.g. Visa, MC, AE, UPC, ... etc, issued by local European institutions and paid in local currencies Euro.  There are 1000s of institutions to choose from if they use Visa, MC, AE.  On the other hand, currently there are about 100 institutions in Europe which issue UPC transacting in Euro.  Although UPC is not as convenient as using Visa, MC, or AE, at the present time,  but it's OK if you only use it at larger stores or more prominent companies, places.

The least preferable option is to use UPC issued by domestic Chinese institutions.  Not only you'd have to pay not-so-preferential exchange rates, you'd also be subject to all those hefty, predatory, inherent fees, charges, and markups, depending who the issuer is.  One would have to be ignorant, lazy, and/or stupid enough to do transactions in this way.  But I am afraid that many, or most, Chinese tourists are doing things this way.