That's because back when TF2 was released, games were still transitioning from print media to digital media. This meant that official guides were usually written in physical print form (since internet speeds back then were slower). Unfortunately, with TF2 being a complex game, this meant there was a steep learning curve unless you were able to get your hands on the printed guidebook.
Here is the TF2 guide you'll ever need, and you'd wish you had this back when you were new in the game:
Click on the image below. Go to Page 269 to find the TF2 section
Here's some pages
First page:
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