This means that only products get shown at the search result page which also have the search term in their title.
We dont want that anymore but unfortunately no matter what i edit in the main-search.liquid code the URL doesnt change. Do you know if I need to change other data and if so, which one and how?
I appreciate the context that you have provided us as that is very helpful. You can duplicate your theme by navigating to Online store > Themes > Actions > Duplicate. I took a screenshot from my test store to show you the section I am referring to:
You will want to create a duplicated version of your theme to have a saved version of the online store. Once you have successfully duplicated your theme, you can attempt to revert the coding changes by Rolling back to an older version of your theme. You can roll back the changes on your theme by navigating to Online store > Themes > Actions > Edit code. When you are editing the code of the theme, you can select the drop down titled Older versions.