Glossary
Web part
A web part is a component you place on a SharePoint page: a news feed, a document list, a chart, a form. SharePoint ships dozens of them, and custom ones can be built with the SharePoint Framework.
Page authors add them from a picker while editing a page. A web part renders nothing until someone deliberately places it, which is why deploying one tenant-wide does not change any existing page.
Before commissioning a custom one, check the built-in ones and JSON list formatting: between them they cover far more than most people try, and they cost nothing and need no deployment.