The modern experience in SharePoint provides you with several widgets or web parts that you can add to improve the experience. Shown here is a screenshot of all the components (client web parts) that you can add on a modern SharePoint page:
While these web parts offer most of what you need to display on a page, sometimes you need to develop custom functionality. In the past, there were several ways you could do this, but nowadays, the SharePoint framework is your best bet. One good thing about building web parts using SPFx is that you get to use the same framework that Microsoft uses to build the out-of-the-box web parts displayed in the preceding screenshot. This means that you get the same benefits, such as responsive behavior and mobile readiness.
SPFx web parts are compatible with both classic and modern pages.
The project is available for download at https://m365book.page.link/SPFx....