Marianne,
The backup procedure does not change your site except to write some records to the Manage Backups table. It isn't possible to destroy your site by taking a backup.
I am assuming that your test domain is in a folder under your live site. If that is the case, you would need to use the Exclude Files and Folders in your live site backup to exclude the test site. If you are sharing a database, you would need to exclude the test site's tables from the live site's backup as well.
You are correct about how to start the restore of your backup. The tricky part is the database settings. It is vital that you use a different database (you must create a new database manually, Akeeba does not have MySQL rights to do this) or change the table prefix for your test site tables. If you don't, then the live and test sites are both pointed to the same set of tables and changes to one site affect the other site.
If you run into any other questions, please let me know.
Dale L. Brackin
Support Specialist
English: native
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