In SharePoint Online we can deploy the app directly to the developer site using Visual studio, but we need to add the app in app catalog for installing the app in Team site, Project site, community site, etc.
Follow the below steps to upload a .app file in app catalog.
Go the admin Portal in your SharePoint Online Tenant.
From Left navigation click on the “apps” to go to app catalog
On the Apps page select the “App Catalog”.
Then you will be navigated to the app catalog site.
If you haven’t created the app catalog site before, it will ask you to create new app catalog. Provide the app catalog name and on clicking ok the app catalog site will be created.
Then click on the “Apps for SharePoint” from the left navigation.
On the “AppCatalog” library you can upload the file using upload option.
Once you choose the file click ok to upload the file,
Finally your app will be successfully uploaded in app catalog library.
In this article we have explored how to upload an app file in app catalog.