Create an App Catalog in SharePoint O365 and upload the sharepoint framework solution to App Catalog

In Order to create an App Catalog in SharePoint, Firstly create an O365 tenant and create SharePoint site collection.

1. Create an O365 tenant
Ref: https://developer.microsoft.com/en-us/office/dev-program
Ref: https://developer.microsoft.com/en-us/office/dev-program

2. Create a new site collection.
Ref: https://docs.microsoft.com/en-us/sharepoint/dev/spfx/set-up-your-developer-tenant#create-a-new-developer-site-collection

3. Create an App catalog site, where we can store our custom SpFx webparts and these webparts will be available across all the site collections.
Ref: https://docs.microsoft.com/en-us/sharepoint/dev/spfx/set-up-your-developer-tenant#create-app-catalog-site
4. Open "Central Admistration", Click on "Apps" in the left navigation panel.
https://<yoursitename>-admin.sharepoint.com/_layouts/15/online/SiteCollections.aspx

or

https://<yoursitename>-admin.sharepoint.com/_layouts/15/online/tenantadminapps.aspx

5. After creating the "app catalog" site, In the left navigation panel you can find a library called "Apps for SharePoint" which is the place where we can upload the ".sppkg" files of SPFX framework.