What is the difference between the left and the right file? The right one is opened in preview mode.

Opening another file won't result in a new tab. Instead, the preview tab is changed to the new file - meaning the file currently previewed is closed.

The preview mode is used e.g. when you click on a file in the explorer or open a file through the Quick Open feature (Ctrl + p).

The following settings will disable the preview mode:

"workbench.editor.enablePreview": false,
"workbench.editor.enablePreviewFromQuickOpen": false

For more information on the preview mode, check out the official documentation.