What is the difference between “flat” and “apartment”?

What's "down-on-your-luck" mean?

The phrase "down on your luck" means experiencing a bad situation or having almost no money. Here, it's used as an adjective, which is why it's completely hyphenated! Ex: I don't enjoy watching these down-on-your-luck TV shows. I prefer happy, light-hearted shows. Ex: She's been down on her luck recently. Ex: Charlie has been down on his luck for a couple of years now. He still hasn't found a stable job.

Does "individuals" sounds more formal than "people?" If so, would it be more appropriate to say "individuals" on formal occasions?

It's similar, yes, but! "Individuals" emphasizes singular people, rather than as a whole. So, saying "people" may just mean a group or many people in general, but "individual" emphasizes each person in particular. A good synonym here would also be "each person," but you're right, "individual" does sound a little more formal! Ex: I got everyone individual presents. => One each Ex: Each person had something to say. => Here, you couldn't use "individual." Ex: I supervise individuals in the company.

It looks unnatural without using "by" next to the made. How come does this sentence make sense?

You do not use "by" after made because the subject making the memories is implied (Adele's memories). If the memories were made by someone who was not mentioned, then you would add by after the word "made." The word "by" shows who or what is performing the action.

Why is it "inspiration for" instead of "inspiration of"? Can I use "inspiration of" here?

"Inspiration for" is correct here. "Inspiration for" is used here because this sentence is saying that Elon Musk "gave" inspiration "for" Robert Downey Jr.'s version of Tony Stark. Therefore, "inspiration of" is not correct to use here, and is rarely used in general. Ex: Roses are the inspiration for my painting.

Could "Hit the building" mean "crash into the building" as well as "attack or shoot the building"?

Although the video doesn't show explicitly what happens after the speaker makes the command to "hit the building," it most likely means to attack or shoot the building with gunfire or explosives. Although "hit" can be used to indicate "crash" in some situations, based on context cues it doesn't seem to be the case here. Ex: We have three rounds of ammunition left. Hit the building! Ex: I had an accident today. My car accidentally hit a fire hydrant.