What does "Get dunked on" mean?

Just like the ships, is it common to call a land a female? If so, can you tell me why?

This is a great question! Referring to geographical (like islands or nature) and political entities (such as countries), along with sailing vessels, it was somewhat common in the past to refer to them with feminine terms (her, she). It's not so commonplace now, and neutral pronouns like "it" or "its" are more preferred. However, natural elements are sometimes still referred to as "her" or "she," and this is related to the conception of "Mother Nature." Overall, it's still preferable to use neutral terms like "it" and "its." Ex: The island is famous for its lush foliage and abundant wildlife. Ex: The country holds an esteemed reputation for its industrial achievements.

Which word is more commonly used in English? A candy bar? Or a chocolate bar?

They're both commonly used! Ex: I got tons of chocolate bars during Halloween. Ex: I got cavities from eating too many candy bars.

What's the difference between "back to" and "back in"?

The preposition "to" cannot be used with the verb "to be" in English. You can't "be to" somewhere, but you can "be in" a place. If you wanted to use "back to" you would have to use the verb "to go". Ex: I just wanted to let you know I AM *back in* the city for 2 weeks! Let's meet up. Ex: She WENT *back to* Seoul last month for a client meeting.

In this context, does the word "room" indicates a "hotel" as a whole? Or literally a "hotel room?"

Here it means a "hotel room!" So, instead of indicating the percentage of hotels, they gauged the percentage based on the total number of rooms across all the hotels. I'd say this is because some hotels have more rooms than others. Ex: The rooms in the southern branch are relatively nicer than the rooms in the northern branch. Ex: How many rooms are in this hotel? Ex: We have a total of 300 rooms across all our hotels.

What does "grab a bite" mean? and is it commonly used expression?

"Grab a bite" means to get something to eat from a restaurant. It is a very common expression and is used often! Ex: Let's go grab a bite to eat. Ex: Should we go grab a bite to eat? Ex: We plan to grab a bite at that new restaurant downtown.