Observation Details
Clicking on a manufacturer filter button will filter the product list without confirmation of the action.
Remediation Notes
As used in the filter dialog window already, a "Filter anwenden" button could fix this issue, although for the given structure, this might not be the best option.
The buttons should have appropriate accessible names to describe their purpose. See ___ for an example solution. The success criterion says, the change of content or context can be announced prior to making the change. Having the accessible name set properly, would meet this for screen reader users. For other users it would be best to visually indicate by text. E.g., introducing the filter section by a short text:
<section>
<h2>Geräteliste filtern und sortieren</h2>
<h3>Filter</h3>
<p> <!-- Add description -->
<button> <!-- Filter dialog window -->
<fieldset> <!-- Checkbox Group for direct filters -->That description could be:
"Öffnen Sie die gesamten Filteroptionen über einen Klick auf den 'Filter' Button oder klicken sie auf einen der darauffolgenden Hersteller-Buttons, um die Geräteliste entsprechend zu filtern."
Or you could even set a <legend> inside the checkbox group to properly describe their purpose.