It's currently not possible to search for widgets without first going to the «Plasma Widgets» page, which is rather unfortunate as many users – me included – don't know that and end up not finding a widget we're looking to install. I strongly suggest that we instead mark the different types of things with an emblem or something to show what it is, and include obvious filters in the search result (which can default to applications as I understand that's the most popular use case).
We've gone back and forth on this. Current consensus is that most users of Discover expect it to show apps, and the more non-app stuff it shows, the worse the search results are perceived as being. It does rather beg the question of whether Discover should even have the ability to browse and search for non-app content, if it's something we want to intentionally de-prioritize.