I don't really know if I'm in the right place to ask the question, and I apologize for my poor English.
At the time, Google had this thing called black bar, some liked it, some didn't like it, some didn't like it, and now there's nothing to go directly from a Google search to a YouTube search without copying everything, typing youtube.com into the URL, and pasting again.
It's not something really difficult, but when you have to do it several times, it's long and the workflow can become stupid and unproductive. Some search engines like DuckDuckGo or Qwant have something called a macro, it allows you to repeat the search on other engines, with example the command !yt.
Best Answer
On Google, you can search only on certain sites by using the
site:operator, iesearchterm site:youtube.comto only get results for YouTube.But I think there's an easier solution, called search keywords. On Firefox, you can right-click any search bar on any website and add a search keyword there, on Chrome you can set them in the Settings (chrome://settings/searchEngines).
Once you've set the search keyword, you can simply type "yt searchterm" to search for your term on YouTube.
See also: https://support.mozilla.org/en-US/kb/how-search-from-address-bar