Documentation of lib commands ah10 and tc9548a

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4

Right here:
https://learn.adafruit.com/adafruit-tca9548a-1-to-8-i2c-multiplexer-breakout/wiring-and-test

#define TCAADDR 0x70

TCAADDR is a macro with the value 0x70.

Also on that page:
https://learn.adafruit.com/adafruit-tca9548a-1-to-8-i2c-multiplexer-breakout/wiring-and-test

void tcaselect(uint8_t i) {
  if (i > 7) return;
 
  Wire.beginTransmission(TCAADDR);
  Wire.write(1 << i);
  Wire.endTransmission();  
}

Neither of these are part of the library. They are the sketch code. That's why you won't find them in the library source code.

It's here:
https://github.com/adafruit/Adafruit_AHTX0/blob/2.0.1/examples/adafruit_aht_test/adafruit_aht_test.ino#L3

Adafruit_AHTX0 aht;

aht is an object of type Adafruit_AHTX0. Adafruit_AHTX0 is a class declared by the "Adafruit AHTX0" library, but aht is just an arbitrary name. You could choose any name you like for this object in your sketch. For example:

Adafruit_AHTX0 foo;

is functionally equivalent (though of course you should always use descriptive and meaningful names).

All these things are fundamentals of the Arduino programming language, but if you go looking in the Arduino Language Reference, you might not find some of this because it is not a complete documentation of the language. One of the things that is not well documented is that Arduino Programming Language is essentially just C++, and any valid C++ is also valid Arduino language (though the opposite is not true). So you will be well served in your Arduino endeavors by studying the C++ programming language.

I like this C++ tutorial:
https://www.cplusplus.com/doc/tutorial/
Some of the information is not of use for embedded systems like our Arduino boards, so you'll need to follow your intuition in gleaning the useful information.

There is also some more advanced topics that you might not have an immediate need for. My recommendation is to skim through for an overview, study and experiment with what is interesting, and take note of the rest in case you might want to refer back to it later.

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