You can install Ultimate Forms in your SharePoint Online environment using Microsoft AppSource marketplace. Navigate to the URL and choose to get the app. You will be transferred to your site and will be able to add the app there.

First you will be asked to grant permissions to API access. You will also be able to specify if you want the app to be immediately available to all sites or if you want to add it to each site individually.

When you choose not to make available to all sites, the app will appear in your app catalog. You will then add it to the individual sites using "Add an app" option in the site menu or in site contents.

When you access the app for the first time, it will ask you to grant permissions. At this point the app asks for the delegated permission only. This permission allows you to enter the app and the app will be able to get information from SharePoint on your behalf. Once you are inside the app, you will notice a banner warning you about the lack of a permission grant. This is the application permission that allows the app to access SharePoint on its own, without a user. 

The application permission is used by a variety of backend processes, such as actions, alerts, import, etc. These components will not function correctly until the permission is granted. Note that the requested permission is a Sites.Selected permission. This means that it is granted to each site collection separately. The app only has access to site collection on which the permission was explicitly granted by an administrator.