The easiest option is to add apps from the SharePoint Store directly due to the user interface integration process. However, if you build your own app to upload or obtain one from another online repository, the only real difference is the first step (for the app source). The rest of the steps are essentially the same.

The following steps walk through obtaining an app from the SharePoint Store and adding it to your App Catalog. You will need to be a SharePoint (or Global) admin and have a Microsoft account to obtain apps from the SharePoint app store and add them to the catalog. Let's get started:

1. Navigate to the App Catalog site you created previously.

2. Select the gear icon (Settings) and select Add an app:

3. Select the link for the SharePoint Store:

4. Locate an app to purchase and install and select it.
5. Click ADD IT (if it's a free app) or select Buy it or Try it (paid apps) to make the app available to your tenant. Some apps may be free to add, but require license keys or other subscriptions to activate, so ensure that you read the license terms. If the app needs to be purchased, you'll be prompted to sign in to your Microsoft account:

6. Click Continue to confirm the acquisition process.

7. Ensure the Add this app to App Catalog checkbox is selected so that it can be installed in your organization. If your app requires additional licenses, you can go back and add those later. Click Return to site:

8. You'll need to trust the newly added app. Read the terms and conditions and any privacy statements. If you accept the terms, click Trust It. You must trust the app in order to make it available to SharePoint sites:

Once the app has been installed into your tenant, it can be deployed or added to sites.