Laptop upgrade by computer speci?

Are computer specialists able to change (upgrade) the hardware of laptops, possibly through new graphics cards etc? Or then connect the graphics card externally through nh bissl cable work?

No, because mostly everything is soldered on.

With some laptops, you can actually replace the graphics card if you may need more graphics power. Otherwise, it is already over with laptops. It's different with tower PCs. You can exchange almost anything.

Is there a way to connect a graphics card externally (without this expensive "graphics card box" that can be connected via USB c)?

The processors in laptops can usually not be replaced because they are soldered, the same is usually the case with graphics cards.

If the laptop has USB with Thunderbolt (and only then) you can connect an external graphics card.

Most graphics cards require more power than a USB port can offer, which is why such cases, which usually also include a power supply, are usually necessary.