Hello everyone ;)

I am new at odoo, and I would like to add some code to the sales(http://url/shop) page, however, this is on python(And I have been working only on regular PHP) I see no place to add it. Then I found a website_templates.xml file, where script files are loaded and outputted HTML code looks the same. I have added my test string there but got no response after restarting odoo and shift+f5 on chrome. Here is the screenshot from Linux - https://prnt.sc/sd2t9h So, as you can see, it should be between the customize and ace javascript. Not only this test string is not in between, but it can't be found anywhere in the source code. Screenshot - https://prnt.sc/sd2ukd
EDIT: changing text in source does not change it on site

I would like also to know, how I could change a picture of a product on the sales tab on a mouseover(Problem is that, I would need a database connection, so I will know the image name to load)