there are two ways to install jquery library in your webpage.
The jQuery library file should be inside the <head> section, see the below example code.
There several (Three)ways to include jquery in the web page
Most of the people use to write jquery code in web page inside the head of the page.
write jquery script any where in the page like inside of the header or inside of the body or external this method is recommend to use, if you want to apply for particular element that time you can use this method.
It's a most common and very useful method use to write if the code is heavy(lengthy) and insert individual modules code in a single file that time it will be the useful method.
Jquery has some basic document structure check the picture differently you will understand some basics like how to start and where to write if you want to write code inside of the file you should mention jquery script type <script></script>.
Here some are the reasons why do people really prefer jquery.