1) XStore theme does not have the search by tags. If you want to get this option I would recommend you to look for additional plugin for the advanced search or submit additional customization request to codeable team.
2) If you use keyword “fish” you’ll get all the results (all the products) that include the “fish” in their titles or description. If you want to search by some category you should choose the category in the drop-down http://prntscr.com/mq9aez and also enter the keyword in the search field. Then you’ll get the products from the fish category that have your keyword. Link that you provided is the link directly to the category, it’s not the link of search results and could not be the link of the search results because of WordPress URL structure.