Aside from the main website, I have noticed that it is hard to find documentation for specific Arduino add-ons, such as (This is just one example):
Is this fairly normal or are you just expected to "just know what to do" with every Arduino component after a while? I realize that the IDE does have samples, but they don't always help.
Where is all of the Arduino Documentation?
So far as I know, the Arduino name is owned by the Italian company Arduino AG.
Arduino AG provide documentation on the website arduino.cc for the products they manufacture.
Arduino AG have no responsibilities for documenting products designed and manufactured by other businesses.
There are many other businesses that design and, in some cases, manufacture products that are designed to be compatible with or to work with products manufactured by Arduino AG. They don't need permission from Arduino AG unless they are using the trademarks or other intellectual property of Arduino AG. In addition, Arduino AG make many designs available under a permissive licence that does not give Arduino AG any control over or responsibility for documentation of cloned or derivative products manufactured or marketed by other businesses.
Examples include Adafruit, Sparkfun and many others.
These businesses provide documentation for the products they manufacture.
Some other businesses might choose to manufacture products without providing documentation or without providing documentation in English. For example a small Chinese manufacturer might only provide documentation in Chinese or provide none at all, expecting users to make use of documentation provided by the community or by the designer whose open-source designs they are using or whose proprietary designs their product is functionally equivalent to.
Many hobbyists and small-scale makers provide documentation for use of their designs for products.
I'd like to add to what JRobert said of finding the information the "hard way"(i.e. looking into the datasheet of the chip it used, and understand how it works from schematic).
In the Arduino world, a lot of hardware/shields are so called Open Source Hardware, meaning that many companies can produce the same hardware with the schematic that is released under Open Source hardware license. China happened to be the country where it has the complete value-chain from components, to manufacturing and cheap labour. They are good in producing such products at large quantity and selling at low price, and they are not necessary good in developing original product, and certainly not in documentation (especially in English). These products are not necessary China "knock-off" as many people call it.
Since it is open hardware products, there are multiple vendors producing it and there are many information available on the internet, and take the example you provide, it is "1602 LCD Keypad Shield", and if your google it with this phase, you will soon find this pdf which is not exactly from the same company of your shield, but it is remarkably similar that you can try.
This applied to many hardware/shields that are available in the market. I hope this provide a general guidance and an "easy way" to find the information in your tinkering journey. Of course, if you want to learn more, go dig out the datasheet and schematic as JRobert mentioned.
To be fair, you're really talking about non-Arduino (capital-A) parts from a multitude of suppliers. And yes, the documentation varies in its depth, quality of translation if it wasn't originally written in English, and even availability at all. Taking the part you linked (from a retailer that storefronts for sellers (sometimes not in this country), who sell from component builders (often not in this country, who build with of parts from supplied factories definitely not in this country, ..., you get the idea. There may or may not exist any documentation specifically for this LCD/keypad combination shield.
But just by reading Amazon's seller's part description, I found that it consists of (at least) a 1602 LCD module and an HD44780 LCD controller chip, for both of which I can quickly find and download datasheets. The sixth image to the left of the description is a schematic of the board which shows that the left, right, up, down, and select buttons are 'multiplexed onto an output called AD0. I might have confirm that that is - or isn't - the Arduino's Analog-0 pin, but in any case, now I know what I have to write to decode which button was pressed from the board's analog output.
All this, and the part costs under US$7.00. For that price (and an assumption that these shields aren't being made in huge quantities), a maker couldn't possibly be able to create a custom datasheet for the shield as a whole, but they did supply a schematic, and their board's components are identified, and those components do, in fact have datasheets online.
Slightly awkward, perhaps, especially if one is thinking in terms of parts like MCUs such as the Atmega328 family, that are made in huge enough quantities that its datasheet runs to nearly 700 pages of engineering-ese. Still, the information is there and accessible, without requiring a way lot of digging.
That's part of being an experimenter.
With this particular board, you're quite fortunate in finding a supplier with an actual website and description: HiLetGo LCD1602 Input/Output expansion board LCD Keypad Shield for Arduino. The fact that the photos of the HiLetGo site show the DFRobot logo — the company that hcheung already referred to — shows just how radically open Open Hardware can get! In this case, DFRobot has pretty decent documentation: LCD KeyPad Shield For Arduino: SKU DFR0009 - DFRobot — but even they don't link to the official LiquidCrystal documentation, but a much older archived version.
The Arduino team admits that they “… never entirely got to grips with Arduino documentation”. Their own "Find in Reference" tool in the IDE will sometimes dead-end to error pages on the arduino.cc site. For example, "find in reference" for Serial will take you to a local copy of the documentation that has broken links for the Arduino Leonardo documentation.
It's not always the case, but typically if an Arduino shield costs less than $10 on Amazon then the cost of "just know[ing] what to do" is passed on to you, the user.
