I thought "high" can only be used when you're on drugs, but I guess not?

What's "have come to" mean and when's it used?

"Have come to" means to result in something or make a decision. "Have" is part of the present perfect tense, but "come to" can be used in other tenses too. It's usually used at the end of something or when you reach the result of something. Ex: We've come to the conclusion that Anne is right about this. Ex: I'm confident that we have a good chance of winning the elections as they come to an end. Ex: The research has come to be useful.

I don't understand the meaning of "at scale" well enough.

"At scale" means "at a large enough amount to make an impact or solve a problem". She is saying that Chipotle's sustainability efforts are enough to make a big impact on the environment. Ex: Our business hired more employees so we can operate at scale.

Can I just use "early" instead of "early on"?

In this context, it would be better to use early on. This is because it references to time and means early stages of an event; the beginning of something If you would just use "early" it doesn't really give a reference of time. In this video, Bill Gates is saying that in the early stages of a virus, we need to be more prepared; we need to have people who are trained to deal with it, run simulations on what to do if another virus should happen, etc. Here are more example sentences with "early on" being used. Ex: The relationship between the two was great early on. Ex: Early on, the play was really boring.

I saw this expression "sleep in" in the movie Home Alone. What's it mean?

To "sleep in" means to sleep or stay in bed longer than usual! Ex: I slept in today. It was so nice getting up later than seven am. That's when I usually get up. Ex: On the weekends, all my friends like to sleep in. So we prefer to go out at night.

Why does he use 's and say "friend of Rachel’s", not "friend of Rachel"? I know that I should put 's when I'm saying "Rachel's friend".

He says "friend of Rachel's" because something used to follow after the person's name (here, "Rachel") that is now omitted. In modern English, it is not known what used to follow after someone's name in this phrase, so it does not make sense to use " 's" here. Therefore, "friend of Rachel", or better, "Rachel's friend", should be used. However, both "friend of Rachel's" and "friend of Rachel" are considered grammatically correct.