To summarize the .wav specification u/turniphat mentioned:
The first 4 bytes tell the computer "Hi, I'm a multimedia file. Please treat me accordingly."
Bytes 5 through 8 tell the computer "Here's how long I am." This is the answer to your question - one of the first things files of any kind will do is tell the computer how big they are, precisely because this is something the computer needs to know.
Bytes 9 through 12 tell the computer "Specifically, I'm a .wav file."
Bytes 13 through 36 tell the computer "Because I'm a .wav file, here are some things you need to know about me. Like, what's my bitrate, am I stereo or mono, how many channels do I have, etc."
Bytes 37 through 44 tell the computer: "Okay, the actual data's coming now. Just a reminder: this is how big it is."
Bytes 45 through whatever number the previous 44 bytes told us are the actual sound itself.
As for why the computer treats 1001 as 9 instead of as 2-1, because at a very fundamental level the computer isn't reading the data bit by bit; it's reading it in chunks (sort of like taking steps two at a time). By default, the chunk size is the "X" that they're talking about whenever they refer to an "X-bit system" or "X-bit architecture", but if a file is encountered, its directives on How to Read This Kind of File take over. So it isn't seeing it as a sequence "1-0-0-1" and trying to figure out where to break it; it's seeing it as a gestalt "1001" (really, "00001001") and treating it as a single unit. If you wanted a 2 and then a 1, you'd need two different units: 00000010 00000001.
Tl;dr: Files share information about themselves to the computer when they're loaded. One of the things they share is how big they are, and another is how many bits of data the computer should read from them at a time.
bulbaquil t1_je2yzza wrote
Reply to comment by turniphat in ELI5: If digital data is stored in 0s & 1s, how does the reader know how many of the digits to take into consideration? by distinct_oversight
To summarize the .wav specification u/turniphat mentioned:
The first 4 bytes tell the computer "Hi, I'm a multimedia file. Please treat me accordingly."
Bytes 5 through 8 tell the computer "Here's how long I am." This is the answer to your question - one of the first things files of any kind will do is tell the computer how big they are, precisely because this is something the computer needs to know.
Bytes 9 through 12 tell the computer "Specifically, I'm a .wav file."
Bytes 13 through 36 tell the computer "Because I'm a .wav file, here are some things you need to know about me. Like, what's my bitrate, am I stereo or mono, how many channels do I have, etc."
Bytes 37 through 44 tell the computer: "Okay, the actual data's coming now. Just a reminder: this is how big it is."
Bytes 45 through whatever number the previous 44 bytes told us are the actual sound itself.
As for why the computer treats 1001 as 9 instead of as 2-1, because at a very fundamental level the computer isn't reading the data bit by bit; it's reading it in chunks (sort of like taking steps two at a time). By default, the chunk size is the "X" that they're talking about whenever they refer to an "X-bit system" or "X-bit architecture", but if a file is encountered, its directives on How to Read This Kind of File take over. So it isn't seeing it as a sequence "1-0-0-1" and trying to figure out where to break it; it's seeing it as a gestalt "1001" (really, "00001001") and treating it as a single unit. If you wanted a 2 and then a 1, you'd need two different units: 00000010 00000001.
Tl;dr: Files share information about themselves to the computer when they're loaded. One of the things they share is how big they are, and another is how many bits of data the computer should read from them at a time.