Isn't Shenzhen from Guangdong? Why do most people speak Mandarin?

Fool, haven't you been to Shenzhen? Shenzhen is an immigrant city, where people from all over the country work and live, and the proportion of Cantonese people is not large. Just like a unit, there are ten employees from ten provinces in China. Only one is from Guangdong. How can you speak Cantonese? We must speak Mandarin before we can communicate, if we all speak dialects, or if you speak Cantonese. So what do I do?