I would like to test my website before transferring the domain or pointing it to the web hosting server. Can I access my website data at XXXX.your-vhost.de?
No, that is not possible. You must always create a domain in your web hosting package so that you can store your own website content and access it via the permalink, for example. The domain does not need to be transferred for this. You can find more information here.
Which features can I use in web hosting without a domain?
Web hosting packages do not include a domain. This must be booked separately and is a prerequisite for many web hosting functions. Without a domain, you can only create databases and cron jobs, use PHP processes, memory limits, and storage space.
Which web hosting features can I only use in conjunction with a domain created in web hosting?
A domain is required to use all web hosting functions. Only with a domain can you publicly host your own websites, activate Varnish, and create mailboxes and forwarders.