Explorer View
If node_modules is hidden in your explorer view, then you probably have a pattern that’s matching that directory in your settings. Go to settings, search for files.exclude and ensure that you haven’t excluded the directory in user settings and/or workspace settings.
Search Results
If you want search results to include node_modules by default, you’ll need to remove the pattern from the search.exclude setting. Note that search.exclude inherits all glob patterns from the files.exclude setting.
If you simply want specific searches to include node_modules, but don’t want it included by default, there’s a gear icon that you can toggle in the search view (go to the search view, toggle the search details using the … icon, click the gear icon in the “files to exclude” input field).