Data has a type. For example, this answer is a text type, known to developers as a `string`. A number is a different type. There are several number types, based on what it is you're storing (`int` for integers, `uint` for positive integers, `long` for bigger integers, `float` or `double` for decimal values, ...).

Why do data types matter? Well, it's sort of similar to why we have file extensions.

Based on the file extension, the file data gets read in a different way. You can easily test this by taking an .mp3 file, changing its extension to .txt, and opening it. You'll see that your OS opens your text editor, not your media player, because it bases its choice on the file extension of the file you're trying to open.  
  
In that text editor, you'll see characters. It's not readable, but as far as your text editor is concerned, this is all valid. You can remove some characters, add some more, and save the file.  
If you now change the extension back to .mp3 and play it, you'll notice that the file is damaged. Maybe it's totally broken, maybe it only has a small glitch in it; this depends on what you changed and how you changed it.

Any file is really just a long sequence of binary digits. And how we interpret those digits is at our discretion. ASCII text encoding is one of these interpretations. It uses a character library of 256 characters, which means that it can get away with using a number (0-255) to denote a single character, and this number handily fits into the space of a single byte. So when a text file is opened, the text editor takes the file data, takes the data one byte at a time, interprets that byte as a number (which inherently ranges from 0-255, and then shows the character for that number.

As an aside, I could write my own file data interpretation. For example, I could use a file's data to generate an image which is 1px tall and is as many pixels wide as the file has bits, and generate a black pixel for every `1` and a red pixel for every `0` in the file data. It's a bit of an odd system, but perfectly viable if I choose to make an application that generates these kinds of images.

Or, alternatively, I could store my students' pass/fail on an exam using a simply stream of bits where `1` is pass and `0` is fail, and I know the order of my students based on some other data I've stored somewhere else. This is a highly efficient storage mechanism in terms of data size, but it is also very inflexible in terms of storing additional information (e.g. absentees).

Back to ASCII. You can do this exercise by hand using [an ASCII table](https://www.asciitable.com/) to look up character values. If your file's first byte is `0100 0001`, which is `65` in decimal, the text editor will give you an `A` character.  
Following this, you can figure out what 5 letter word I wrote here in ASCII: `0100 0001 0101 0000 0101 0000 0100 1100 0100 0101`. If you take each byte, convert it to a number, and look up what character that is, you'll see that the 5 characters are `APPLE`.

One very important thing to notice in that linked ASCII table is that it doesn't just contain letters (a-z and A-Z), it also contains all digits (0-9).

You'll notice that this answer focuses on text, not numbers. There's a clue in your question:

> However after typing in the number 65535 into **a text document** and looking up its file size (the "information" about the text doc) I got to see that the used storage for this document was actually "5 bytes"

The data you entered was parsed as text, not as a number. You did not store the numerical value of `65535`, you stored the individual **characters** (i.e. text) of `6`, `5`, `5`, `3`, and `5`. Using the ASCII table, this means that your file contains `0011 0110 0011 0101 0011 0101 0011 0011 0011 0101`, which is the same as the `APPLE` example, but this time using the characters `65535`.

[Here's an online converter](https://www.rapidtables.com/convert/number/ascii-to-binary.html) so you can play around with this (you can press "Swap" to switch converting to or from binary.