See the circled Icon that looks like a stack of images? It toggles between editing all selected images ("variants") at once and only editing the currently shown selected ("primary") image. Simply deactivate it to get the behavior you want.
Image Alchimist's article on "Capture One Variants" states:
First, as I told you, the Primary Variant is always in the action. Whatever you do, the Primary is affected. When you move the sliders of a tool or change a value, it only affects the Primary, not the Other Variants. This way you can individually adjust an image within a selection; you do not have to break up a selection.
Second, when you copy adjustments with Capture One from one to other variants you always copy from the Primary to the Other Variants. So the Primary is the source, and the Other variants are the target. Therefore, select your Primary carefully within a selection when you copy adjustments.
Third, actions like delete, export, move, and clone affect all selected variants equally. Also when you apply a preset or style (predefined adjustments), all variants are affected equally.
But, with the toggle “Edit All Selected Variants” switched Off, only the Primary is affected.
This allows you to export or move just a single variant while retaining a selection, or apply a preset to only one variant: the Primary.