An electronic device involves lots of different chips, components (resistors / capacitors), connectors, and so on -- maybe dozens or even hundreds of parts. The parts have pins or wires sticking out of them that need to be electrically connected. (Some chips have a couple dozen connection pins.)

A circuit board is a sturdy plastic / fiberglass surface on one side, copper on the other side [1]. A "new" circuit board has a solid copper surface on the back, coated with a light-sensitive protective chemical.

A solid surface is no good though. As a circuit designer you don't want everything connected to everything else, you want to make connections between specific points only.

So to get the connections you want, you make a shadow over the parts of the copper you want to keep, then turn on a bright light, then wash the copper with acid. The acid dissolves the copper, but only where the light destroyed the protective chemical.

Now you just have to:

• Put your components in the right places on the fiberglass side
• Drill holes for the wires / pins
• Glue ("solder") the wires / pins to the copper with a tiny bit of molten metal

After soldering, the parts are mechanically locked in place, electrically connected where they should be and insulated everywhere else.

There are many circuit board customization companies that offer these services for very cheap prices. You send your circuit design files to the company, and they use specialized robots to wash the copper, drill the holes, and also print directly on the board whatever graphics / labels you want ("silkscreening"). They ship you the finished circuit boards by mail / UPS / Fedex like any other online delivery. For an additional fee, another specialized robot can place the components and solder them for you ("pick and place") [2].

You can buy one or a few circuit boards with a unique circuit design for a prototype or a hobbyist one-off. Or you can order hundreds or thousands of identical boards at a time, if you're making a mass produced commercial product.

[1] Circuit boards can also have multiple layers.

[2] A lot of modern electronics use components ("SMT") that have very small, closely spaced connection points. These components are really designed to be soldered by robots; it's difficult for a human to solder them by hand.

Each sound card speaks a different "language." In the days of MS-DOS your program had to talk directly to the sound card, so you had to add the "language" of the specific sound cards you wanted users of your program to be able to use.

With more complicated OS's (Windows, Linux, OS/2), the OS speaks a single "language" to programs, and under the hood there's a "translator" (device driver) that works for each sound card.

Basically it stopped being a problem when PC's started using OS's that would "do more" for programs, including talking to the sound card. In turn this new kind of OS was enabled by other underlying trends:

• Computers gained more memory, more disk space, and more processing power, there's enough spare performance to add extra layers/complexity to the OS and software
• PC CPU's gained the ability for an OS to run in a "privileged" mode and "be in control" of the computer, isolating programs from each other and the hardware
• Larger floppy disks (1.44 MB) and CD-ROM's made it easier to distribute large pieces of software
• Microsoft made deals with PC makers for Windows 95 to come standard with new PC's
• A larger market [1] means it becomes financially feasible for a company to pay higher dev costs to create more complicated software, as they can predict they will sell enough to recover the dev costs and make a profit.

[1] Before 1995-ish, PC users were kind of like MMO players or VTuber watchers today, a large community but a bit niche / nerdy and not quite mainstream. From 1995-2005 PC's became more like smartphones today, considered essential devices for the vast majority of the population (at least in the US and other advanced countries).

An airplane engine is basically a giant fan that pushes a huge amount of air backwards, which pushes the plane forward and upward.

As you go higher, the air gets thinner and thinner. As you go toward space, airplane engines lose power as they have less and less air to push. (And if you have a jet engine like most modern passenger aircraft, it needs a certain minimum amount of airflow to run at all.)

> if a government wants to borrow money it issues bonds at a fixed interest rate

Government bond interest rates are not fixed. They are set at auction.

Government: Here's an IOU. It says "I pay you $100 in yearly installments over the next 20 years -- US government" and this crowd of bidders has assembled to buy it today.

Auctioneer: Do I hear $50? $70 from the man in the red shirt. Okay, I hear $75 from the woman from BigCapital Megafund. JP Morgan, with your hand in the air, $80. Last National Bank, $86. $86 from Last National, any other bids? Going once...Going twice...SOLD to Last National Bank."

Then Last National Bank pays the government $86 when the auction ends. According to the IOU, the government pays Last National Bank $5 a year for the next 20 years.

Except the auction process isn't a live auction, it's done by everyone sending their bids to a government computer system that can handle thousands of bidders. After the deadline for submitting bids, the computer calculates the auction results.

• A put option is basically an insurance policy against the price of a particular stock going down (for people who want to sell that stock in the future).
• A call option is basically an insurance policy against the price of a particular stock going up (for people who want to buy that stock in the future).

In the US, the options (i.e., the insurance policies) have standardized terms, and can be bought, sold, and used through most online stock trading services.

Options are more like gambling than investing, and it is also much easier to lose large amounts of money. There are some legitimate uses for options, but they don't apply to 95% of the people reading this. Unless you understand very well how options work, you should treat it as similar to buying a lottery ticket or going to the casino and betting on dice:

• Spend only money you can afford to lose
• Realize you're probably doing business with someone who's mathematically proven that you'll lose money on average
• Realize that it's very easy to make very expensive mistakes
• Don't be surprised if you lose it all
• Consider it entertainment purposes only, not a strategy for reliably earning money