ᐅ Is a crack at least 2.00 meters long in Q2 concrete considered normal?

We have some cracks in our terraced house that I would consider minor, basically hairline cracks. However, one is causing me some concern... In the stairwell, there is a crack over 2 meters (6.5 feet) long that does not look like it is only 2 centimeters (0.8 inches) wide. See the photos. Can anyone tell me if this looks harmless, or would you recommend consulting a building expert?

Should this be concrete? I would have said plaster.

What is behind the plaster? Is it actually concrete or some other building material?