I was working on something else other than the airplane and somehow the parts became unselectable in the 3D viewport. I cannot select any part on the airplane in the viewport but I can choose anything else that is not part of the airplane. It's only happening in the airplane's collection. I can select the parts as normal using the outliner, but I cannot do it in the viewport. I don't know if this is a bug. What could be the issue?
Most probably the collection is marked as
not selectable in the outliner.
Have a look at the icons on the right handside of the collections line in the outliner and switch the
selectable icon to
This is the icon, make sure it is enabled on the collection and all objects further down the hierarchy:
If you cannot see this icon in your Outliner, go to the Filter menu (the funnel symbol) and enable it in the Restriction Toggles: