Here’s the landing page if you just go to www.search-lemmy.com. I’m assuming that drop-down that you’re referring to is your home instance selector. Since you’re on lemmy.world I suggest you set that to well, lemmy.world. Then you can do your search and all of the results will take you directly to that post on lemmy.world (or whatever you set as your home instance).
Now you can also see that Find Communities button in the top right, you can click on it and it’ll take you to a similar page but instead of returning posts for search results, it will return a list of communities, based on how many matches it found. (as if you did a search on the normal page but instead just counted the number of results per community).