Deployment mechanism (App Stapling) allows you to deploy an app to any site on your tenant / farm. It's available for Rich Forms (v3.1.27.28 and later) and Theme Designer (v1.2.2.7 and later) in SharePoint Online and SharePoint Server 2019.
Once the Add-In is installed on App Catalog site (i.e. {tenantUrl}/sites/AppCatalog) 'Deployment' option is available by clicking the eclipse on the Add-In.
On Manage App Deployments page you can configure deployment to specific site collections, managed paths, and site templates.
After successful deployment Add-In is available in site contents and can be used.
Note: Forms of Document Library cannot be customized by Rich Forms Add-in deployed through 'App Stapling' due to technical limitations. Rich Form Add-In should be installed directly to these sites to customize Document Library's forms.