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A backup archive is a full backup of your site: files, and database content. It does not add / import content to an existing site. It replaces the site.

As a result, the restoration script cannot assume that the wp-config.php has any significance. It does not use it. At backup time, we store the URL to your site and its database connection information (unless you explicitly specify otherwise).

During the restoration we check the URL we are getting restored to, as reported by the server. If the URL matches the site the backup was taken on the database information is pre-filled. If not, they are left empty. This is a deliberate feature. It prevents people from doing silly mistakes, like backing up subdomain A, restoring on subdomain B, and forgetting to change the database connection information which would result in both subdomains sharing the same database, with disastrous consequences for both sites.

Since you are restoring a backup taken on a different site, the URL the backup is being restored to and the URL the backup was taken on are different. Therefore, the database connection information remains blank. You need to enter the database connection information yourself.

When you want to transfer, or clone a site you do not – and, ideally, should not – create a new site and import the backup archive. Akeeba Backup is perfectly capable of restoring on a blank site as explained in our documentation and our video tutorial: https://www.akeeba.com/videos/1215-akeeba-backup-wordpress/1651-abtw05-restoring-site-new-server.html  This makes sense, since Akeeba Backup is backup software. If you cannot restore on an empty server, what you have is not a backup, it's a content export. Those two things are very different and should not be confused. A backup includes all files and all database content, allowing to create identical clones of a site on any server. A content export only includes some database data and, sometimes, some files, allowing you to add whatever it considers content* to a different, already set up and running site.

* "Whatever it considers content" is the operative condition here. There is no universal definition of what constitutes “content”. Depending on someone's definition, media might or might not be content, block templates might or might not be content, WooCommerce products might or might not be content, WooCommerce orders might or might not be content etc. This is why content export and import is never satisfactory to anybody unless they happen to find content transfer software which sits in the perfect intersection between their use case, what is technically possible, and what the software can do.

I am going to install a blank website in instawp, get all database infos, upload jpa in root and restore with kickstart?

This is the best way, yes.

What you are doing is essentially the same thing as long as both the version of WP you installed and the one you backed up are identical.