Is this Udemy course in Mandarin, are Mandarin and Chinese very related, and would Mandarin have any use in Hong Kong? Before moving to Hong Kong in 2 months, I want to learn Chinese and when considering Udemy, I came across a course Chinese Made Easy L1: Understand 65% of Chinese in 10 hours.

However the description says "Chinese (Mandarin)". Does this mean that it is a Mandarin course? Are Mandarin and Chinese very related?

If this course is teaching Mandarin, and if Mandarin is a way different language than Chinese, would it have any use in Hong Kong?
I checked the website.  Yes, it teaches Mandarin.

Mandarin is the formal language used in Taiwan and mainland China (including Guangdong).  Mandarin is written using simplified Chinese in mainland China, while written using traditional Chinese in Taiwan.

Cantonese is used in Hong Kong and Guangdong, written using traditional Chinese in Hong Kong.

"Chinese" includes Mandarin and dialects used in some cities.  Among them, Cantonese is significantly different from Mandarin, and most of movies released in Hong Kong need to be dubbed so audiences from Taiwan and mainland China can watch them.