system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do insurance companies make money? Who insures insurance companies?

If I pay $150/month and total my car after 12 months, I've only paid in $1800, yet I will get back ~$10,000 (depending on the vehicle, of course). How do insurance companies make a profit from that, and how are said companies insured? Can they insure themselves? Who do they pay? Wouldn't it be like writing a check to yourself? Also, how do people start up their own insurance companies?
gpt
The main thing you need to know about how they function is this:

They are INVESTMENT ENTITIES.

Nominally, the thing most people see is the spreading of risk, pooling money from lots of people to transfer that catastrophic risk from the one person who loses a home, wrecks a car, or gets cancer, to the group of people that all buy insurance.

But what does an insurance company do with that pool of money in the meantime?

See, the company has a bunch of money called a "surplus" which is mandated by law based on line of business (health, auto, home, life), AND it has a bunch of money from what people have paid in but hasn't been assigned to pay a claim yet, AND it has a bunch of money "reserved" for claims that are submitted, but not yet paid out (like worker's compensation claims, etc.).

This money is INVESTED.

You may be surprised to know the that the biggest buyers of bonds are insurers. (because of regulation, the amount of stocks bought by most insurers is pretty minimal).

Also, note that "life insurance" companies are primarily about selling investment products to customers; the products that pay out based on insured deaths are a small portion of the total sales.

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As to "who insures them," you need more to the question, as in "insures them for what?"

Your typical "personal lines" (i.e. auto, home) insurer will have its workers' comp, health, life, and its buildings' and infrastructures' property (assuming it doesn't just rent those facilities) with other companies.

And, typically home insurers will purchase reinsurance from global companies to protect against large-scale losses (Hurricane Rita, for example).