Please help. Big confusion about potential. Urgent.?

The word "high" here carries a relative concept. It means a "high potential difference relative to ground". Should the dome carry large amount of -ve charges, the potential difference of it would still be high in respect to earth potential."

If an object carries charges, it will acquire a certain potential from the charges. At the same time, the charged object will also raise (or lower, if the charges are -ve) the potential of the surrounding space.

That said, charges accumulated on the dome of a van de Graff generator cause the dome to acquire a certain potential.

In terms of the definition of potential, the "potential of the charged dome (assume the charges are +ve)" is the work done required to bring a (+ve) unit test charge from infinity to reach the dome.

yes, you correct. but it is normal positively charged