It snows in Chengdu

First of all, the blocking effect of Qinling Mountains, Qinghai-Tibet Plateau and other high-lying areas is the most important reason.

Secondly, I don't think you understand why Zigong and Chengdu are both in Sichuan Basin, but the snowfall is different!

I think so, too. Chengdu is closer to those higher-lying areas than Zigong, which means it is more difficult for cold air from the northwest to reach Chengdu. However, after entering the Sichuan Basin, the cold air began to sink slowly, without the influence of the tall terrain where the cold air could cool, so Zigong would encounter these sinking cold air.