Is it common to say "100 percent" to exaggerate something in English?

Can I use "completed" instead of "accomplished"?

Good question. "Accomplished" can be used as a synonym of "completed" and vice versa. As verbs there is however subtle nuance between them. "Accomplished" means to finish something successfully, "completed" on the other hand simply means to finish or reach the end of something. Because of this they can't always be used as synonyms of each other. In the case of this clip, "mission accomplished" means that they finished successfully. Changing it to "mission completed" means they finished the mission but doesn't necessarily mean it was successful. Ex: I completed my homework. (Finished but I don't know if it is successful until my homework is marked) Ex: I will complete my novel tomorrow. (Finish) Ex: He was very accomplished in his work. (He was successful) Ex: Did you accomplish everything you needed to? (Complete)

Why does dollar have “s” at the end while “a billion” seems singular?

There is an 's' at the end of the word dollars because you have more than one, which makes it plural. You do not use an 's' after the word "billion" when the words "a" or "several" are before the word billion. Ex: He makes several billion dollars a year. Ex: The company has a billion dollars to spend this week. You also do not make "billion" plural if there is a specified amount before it. Ex: She has two billion dollars. Ex: Freddie makes eight billion dollars a year! You can make "billion" plural as long as there is no specified amount. Ex: Billions of people died from the disease.

What's this line mean?

This line means that she'll be by herself for one more day. As each day comes, she's still "on her own" without anyone else there. Ex: Two more days on my own in quarantine, and then I can see people. Ex: You won't be doing the competition on your own, don't worry.

What does "faux Asian" refer to here? Did the speaker say "faux" because of the shape of the letter which looks bit odd?

He refers to the "thank you" as being "faux Asian" because it was meant to look like writing from an Asian language (Chinese, Japanese, etc.), but in reality it does not resemble any written language from Asia. It is also written in English, but is supposed to look like it is not written in English (although it is clearly written in English).

What does "let it go" mean?

To "let it go" means "to forget or not care about something." For example, "She felt she had been treated wrongly, and she wasn't willing to let it go."