I wanted to ask if you can also use the XP-Pen Artist 12 without it being connected to a PC / laptop
No you can't. This is a grafiktablet / graphic monitor, which are never really standalone.
but there are also pen-computer https://www.wacom.com/de-de/products/pen-computers
They work stand-alone. But are much more expensive, because so photoshop and co. You need a lot of power to run.
Of course, wacom products are not intended for normal consumers, it is clear.
But usually it makes more sense to have a separate computer anyway if the circumstances allow it.