Whenever you search for something, and it redirects you to search.yahoo.com, you probably have a browser hijacker on your device. So, to remove Yahoo Search from Safari, first, go to the 'Preferences,' select the 'Extensions' tab. Now, choose an extension you want to delete, and tap on the 'Uninstall' button next to it. Now, tap on the 'General' tab and choose what you want Safari to open on start-up, new windows, new tabs, and homepage. Then, go to the 'Search' tab and select the search engine of your choice. You should also remove the adware/malware controlling the settings to stick again to Yahoo search.
See also - Why Yahoo Mail Search Not Working