Focus on 3 behaviors – create, view, and edit items in a list. Render one or more behaviors on a form. Starting with required tools, create a SPFx extension project, review provisioned components/code, 2 MUST use methods, overriding behaviors, creation and rendering options and finally deploying a form to production. Call REST to add or update items. This PnP Community demo is taken from the Viva Connections & SharePoint Framework Bi-weekly sync call recorded on August 25, 2022.
Form customizers are SharePoint Framework components giving you an option to override the form experience in a list or library level by associating the component to the used content type. Form customizer components can be used in SharePoint Online, and you build them using modern JavaScript tools and libraries.
https://docs.microsoft.com/en-us/sharepoint/dev/spfx/extensions/get-started/building-form-customizer