Companies

SolarCity is already merged with Tesla. 

Tesla doesn’t make just cars, and it’s a more complex company than you can imagine.

SpaceX makes rockets and other things.

Hyperloop One doesn’t have Musk on board. They “stole the idea!!!1111!!!”, but he is not part of the team. -> Hyperloop One - Wikipedia

But instead of Hyperloop he has founded The Boring Company -> The Boring Company - Wikipedia
“an infrastructure and tunneling company founded by Elon Musk”

OpenAI is a research company “associated with” Elon Musk. What is his role inside the company?
Also, the co-chair is Sam Altman of YC. He co-founded with another great entrepreneur.

Neuralink was founded by Musk in ’16 and publicly reported in March ‘17. The product is said to ship in 4 years.

He could make a conglomerate like Alphabet or Amazon which deal with a lot of different businesses. These two companies don’t get enough credit by fanboys (though the stocks are worth >4x than what all the companies owned by Musk could be and Bezos is the second richest person in the world). He keeps things separated because he always bets big -> Elon Musk, of PayPal and Tesla Fame, Is Broke (2010).
So, can you imagine if Tesla ramped up too much debt, produced so many cars at a loss, competitors catched up in 4–5 years, and then a scandal with the proportion of the Volkswagen’s one hit the company?
He’d put in serious danger the project of humans on Mars.

What does it take for a normal person to multi-task at this range?

a) Elon Musk is not normal. It’s like asking “how do I become MVP of the NBA finals if I’m 5′10 and I’m not a basketball player?”. It doesn’t happen. Period.

b) He is not multi-tasking here and there. He is focused on one thing and one thing only -> Succeed. He can’t say “oh, I allocated 3 hours at SpaceX this morning, let me take the corporate jet and see if I can get to Tesla’s in Palo Alto and do 3–4hours of work there”. No, he is exactly and always where he needs to be. Where he makes the biggest difference. Mark Zuckerberg said something like “If I’m not doing the most important task of the day, I’m wasting my time”.

The best advice I can give to you OP(!) is to cut all the carps.

Good is the enemy of great.

You need to focus on the things that have the biggest impact, you need to skim things that are just good, and go where the hard things are.

I get very disappointed when I see a long answer with a lot of upvotes which is not relevant to me -> I lost minutes of my precious time, and for what? To get insipration? to get motivation? JUST DO IT. Don’t read Quora all day long.

Do you think Musk is going to read my answer? lol.
Do you think he cares what they tells to him on Twitter? Well, maybe…
Given that it’s a PR move, very successfull (and IMHO there’s a lot to learn from him and Marc Andreessen in this regard, if you are a CEO).
He knows how important is the perception of his ventures for the good of his companies.
Good PR -> high stock price -> higher ability to raise capital to fuel not-yet-profitable companies.

Could the SVP of engineering at Tesla have the impact he has at doing this task on Twitter? No.
But I’m sure that the SVP of engineering is (one of) the best fits for his role. Who put him there? Probably Musk.

One last point:

As User-11195922798142823543 says he is not alone in this.
 I think it’s naive (don’t get offended) to think that he is a one-man-army.
Every great entrepeneur knows that you need to attract the best talent and drive higher the curve of skill set in your company -> hire better than you.
You must delegate and not micromanage, especially at the scale at which Musk operates.
If I were an employee I’d find disrespectful that somebody thought that the CEO, and the CEO only, did all the work.

Bonus:
Go, check this link, out yesterday, great reading: Neuralink and the Brain's Magical Future - Wait But Why