Adding an app to a site is a straightforward process. As long as there are enough licenses available and it is available in the App Catalog, it can be added to a site. If you obtain an app from the SharePoint Store, it is automatically available to the sites to be added. To add an app to a site, follow these steps:

1. While logged in as an administrator, navigate to the site where you wish to install an app.
2. Click Settings and select Add an app.
3. Under Your Apps | Apps You Can Add, select the app to deploy to the site.
4. Select Trust It.

The app will be available under the site's Site contents menu. If the app contains a web part, you may also be able to edit the site's page, add the web part, and republish the page. Not all apps contain web parts, so you will need to check with your app's administrator guide to see if this feature is available to you.