Brief overview of Chinese-language TV dramas from PRC, Taiwan, Hong Kong, Singapore, and Malaysia
PRC - predominantly Mandarin, a few local channels will make dramas in local languages like Shanghainese (on Shanghai TV), Cantonese (Guangdong TV), and Hokkien (Xiamen TV), but these tend to be of inferior quality
Taiwan - predominantly Mandarin mixed with smaller amounts of Taiwanese Hokkien. A few channels (SetTV and FTV) mainly produce Taiwanese Hokkien dramas, which tend to be incredibly long (500 2-hour episodes aren't uncommon). There are also Hakka dramas (Hakka TV).
Hong Kong - completely Cantonese, though they are usually dubbed in Mandarin. Hong Kong made tons of Mandarin movies before the 1980s but these have been phased out. Many Mandarin speaking actors moved to Taiwan
Singapore - completely Mandarin. Singaporean government is actually openly hostile against Cantonese and Hokkien which many residents speak at home
Malaysia - predominantly Mandarin, with some Cantonese (not really sure about Hokkien?)
I haven't heard of any dramas from Macau - dramas that take place in Macau appear to be produced in either Hong Kong or mainland. I'm not aware of any Chinese-speaking TV dramas from the west.
You can find dramas from all of these places on the Internet. Collaboration is very common, as is dubbing. For many Internet shows like those on YouKu, IQiyi or Tencent the country or region is also not always clear even for modern dramas.
Thank you for this full description. I am aware that Singapore doesn't welcome non-Mandarin Chinese dialects and my cousins that live there are taught Cantonese at home instead of at school. Singapore doesn't allow dramas or movies that are in other Chinese dialects other than Mandarin.