I have started working on SharePoint Online. Where I have to use SPFX.
What is the Difference between SharePoint Framework (SPFx) Web Part and SharePoint Framework (SPFx) Extensions
Web parts allow you to create components that can be added by content editors as content on a page. Extensions add functionality to known areas of pages on an entire site or site collection, for example to add things in the header or footer of every page, or add menu options in a list toolbar. Extensions are often added by administrators.
Both use the sharepoint framework (spfx).
Does it help?