Search in Drupal is built by "Indexing" information from each page or node. Whenever the site "Cron" is scheduled to run, the system will gather any new or updated content and place it into the default Drupal index with just strictly information from the title, and body of the content. No other fields will be indexed.
Default Drupal search isn't amazing. It's not even close to amazing. It's basic and it's free. Utilizing third party search tools is recommended when possible. Also, excluding by content type is not a default option. All content will be searched without adjusting the system with additional modules to force it to exclude content.
Here's some add-ons we suggest or your hosting partner may provide:
Search API Module - This is a Drupal add-on module that makes it possible to connect to the third party options listed below.
Faceted Search - A Drupal add on module that will allow you to choose and search with fields by type of information on the site (content type, term, etc)
Solr Search System - http://lucene.apache.org/solr/ A very complex search system built in Java which requires additional services or hosting. One of the best in terms of giving you good results in an algorithm - and having add-ons for did you mean / misspellings / suggested results / and customizing the weight of types of content.
Google Site Search - Somewhat sunset by Google, you can embed a form onto the page of your site that displays Googles search for your site. Minimally customizable, and can display ads.
Elasti Search - Somewhat new competing with Solr Search. Very similar results to Solr and add-ons. allows customization and like Solr requires you to host it separately.
What if I use one of these alternative search systems and it stops giving me new search results? - Check your cron in Drupal. If it's not processing your search index, any of the above searches will stop giving you updated results.