Search engines like Google offer an "Advanced Search" option that will limit results to a specific website or domain type. Each search engine will have their own version for how to do this, but Google's is below.
To limit to specific sites like .gov, type ".gov" into the "site or domain" field. See this video or the information below for more context.
You can then search for your topic area and retrieve only .edu (or whatever you've limited to) website results.
Notice that the Google Advanced Search generates the tags for your search string: "Site:.org" or "Site:.gov". It can save you from having to remember those tags in the future. A typical search engine (such as Google) will also allow for Boolean and keyword phrasing. Learn more about that here.
You can also copy and paste the url of the website you want Google to find results for (such as if wanting to search for National Geographic or another site for specific articles on a particular topic). Here is an example for an online digital magazine called Undark and the results (notice the results are all from their site):
