How the Digital Economy Enhances the Innovation Capacity of Cultural and Tourism Integration: Empirical Evidence from Cities in the Middle and Lower Reaches of the Yellow River

The digital economy is profoundly reshaping the global economic landscape; however, its impact on the innovation capacity of cultural and tourism integration—particularly within developing regions—remains insufficiently empirically tested. Utilizing panel data from 43 cities in the middle and lower reaches of the Yellow River (2009–2023), this study constructs indices for the digital economy and the innovation capacity of cultural and tourism integration using the entropy weight method. Employing a two-way fixed effects model, instrumental variable approach (using lagged variables and historical interaction terms), and mediation effect models, we empirically examine the impact, transmission mechanisms, and heterogeneity across city scales. The findings reveal that: (1) The digital economy significantly enhances the innovation capacity of cultural and tourism integration, with a benchmark regression coefficient of 0.683 (significant at the 1% level). (2) Mediation analysis indicates two transmission pathways—enhancing development potential of culture and tourism and promoting digitalization of cultural and tourism enterprises—with mediating effects accounting for 15.95% and 2.62%, respectively. (3) Heterogeneity analysis shows a significant negative effect in megacities (coefficient = −1.3726, significant at the 10% level), contrasting with positive effects observed in super-large and large cities. This study elucidates the differentiated pathways through which the digital economy empowers cultural–tourism integration innovation, providing empirical evidence for tailored digital–cultural–tourism policies across cities of varying sizes, with implications for other developing regions exhibiting similar urban hierarchies.