“The reason I acquired Twitter is because it is important to the future of civilisation to have a common digital town square,” Musk wrote in a letter addressed to Twitter advertisers, which he tweeted Thursday.

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From building rockets and brain chips to digging tunnels, here’s what all of Musk’s companies do.

1. Tesla

Musk is best known as the co-founder and CEO of Tesla, which he helped start in 2003. The carmaker has since designed seven electric vehicle models, four of which are currently in production.

Tesla’s headquarters recently relocated from Palo Alto, California to Austin, Texas. Nearly 100,000 people are employed by Tesla around the world, with over 28,000 new hires in 2021 alone.

Tesla is one of six US companies that have reached a US$1 trillion market cap – more than its nine largest competitors combined.

In 2016, Tesla acquired SolarCity, the US’ leading provider of solar panels. Today, Tesla sells four solar energy storage and generation products: Powerwall, Powerpack, Megapack and Solar Roof.

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2. SpaceX

Next up: SpaceX, the aerospace company founded by Musk in 2002.

According to his company bio, Musk is the “lead designer” at SpaceX, where he oversees the spacecraft and rocket development “for missions to Earth orbit and ultimately to other planets”.

SpaceX is headquartered in Hawthorne, California. However the company’s launch site, known as “Starbase”, is located in Boca Chica, Texas.

SpaceX also owns the business Starlink, which provides low-orbit satellite internet services designed for rural areas with unreliable internet. The first batch of Starlink satellites were launched in 2019.

3. Neuralink

You’ve probably never heard of it, but Musk is also the co-founder of a San Francisco start-up called Neuralink. The company’s mission sounds straight out of a sci-fi movie: to connect the human brain to computers.

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Musk is listed as Neuralink’s CEO on Tesla’s website, but his exact involvement isn’t clear. The company previously stated in legal documents that the billionaire “has no executive or management role at Neuralink”.

Through the development of a brain chip Musk has described as a “FitBit in your skull”, Neuralink aims to treat neurological conditions ranging from paralysis to depression.

The billionaire helped found Neuralink to “help those with brain injuries in the near term and reduce AI risk to humanity in the long term”, according to its website. Last summer, Neuralink raised US$205 million in Series C funding.

The neurotechnology company has 245 employees listed on LinkedIn and is ranked No 33 out of 50 on the site’s 2021 “top start-up” list.

4. The Boring Company

Musk is also the founder of The Boring Company, a tunnel construction start-up based in Texas that launched in 2016.

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Musk’s tunnel-digging venture aims to solve traffic within and between major US cities. One of its future projects is a “Hyperloop”, which would theoretically allow passengers to travel at 1,130km (700 miles) per hour. The interior of a Hyperloop tube after the first test of a propulsion system at the Hyperloop One Test and Safety site on May 11, 2016 in North Las Vegas, Nevada.

Beyond Musk’s core business ventures, the billionaire has been tied to at least seven additional companies, per The New York Times. This includes Wyoming Steel, an LLC he has reportedly used to purchase real estate.