I want to set the search box to search not only in product name, search keywords & description but in the product tags for the "typed keyword. I just made the searchbox to search by code too, but could not find how to fix it to search in the tags too…?
Since 'tags' is an add-on and not built into the core of the cart, you're going to have to add this functionality.
To begin with, you're going to have to investigate /core/fn.search.php to include product tags in the search using. Doesn't look like it's going to be a quick and easy task to complete in 5 minutes so if this requirement is mission critical you may be best looking for a custom development service.