OpenProject integration provides a dashboard widget displaying your important notifications, a search provider for work packages and notifications for changes in active work packages.
The account configuration happens in the "Connected accounts" user settings section. It requires to create a personal access token (API key) in your OpenProject account settings.
A link to the "Connected accounts" user settings section will be displayed in the widget for users who didn't configure an OpenProject account.
There also is a "Connected accounts" admin settings section if you want to allow your Nextcloud users to use OAuth to authenticate to a specific OpenProject instance. An admin can create an OAuth app (and get a client ID and a client secret) on OpenProject side in Administration -> Authentication -> OAuth applications.
To be able to periodically check activity in OpenProject (when "notifications for activity in my work packages" is enabled), you need to choose the "Cron" background job method and set a system cron task calling cron.php as explained in the documentation.