... if such an aircraft did miraculously appear, it would certainly get a lot of attention from everyone involved. So what sort of aeronautical details could they usefully get from having a modern commercial transport aircraft to examine?
In my area of knowledge? If I was some one like Barnes Wallace say and I was looking at the interior of a stripped out 737 fuselage I'd see evidence of chemical milling of fuselage skins .The shapes that result give clues as to load paths ,from this you may be able figure out that aircraft are much more dynamic in response to loads than 1940s technology .Round windows with what are called bear/tear straps around them should prevent you repeating the Comet 1 disasters The hydraulic systems of the forties will leap forward as well no more flamable vegitable fluid fires

Pilkingtons Lmtd will figure out how to put gold mesh in laminated glass windscreens much earlier
Materiels will advance as well lighter, stronger aluminiums with less impurities ,plastics [of course]
Not sure what they would make of carbon fibre not sure myself to be honest

Fasteners will change rivets will look the same but will be made of different aluminiuns ,diferently treated ,less bolts that look like the ones you might buy in a hardware store These are replaced by several specialised fasteners which [just to pick one example] differentiate between sheer and tension loads
Less fasteners overall in fact due to integeral milling techniques especially in wing skins
Big one would be 'wet' wings difficult in the '40's easy now due to better assembly techniques and good poly sulphide sealants
Anti corrosive compunds which were unknown then will be seen [not just paints] as will the fact that modern aircraft are bonded [last thing you want on any a/c is an area of higher/ lower electrical resistance]
Wiring will be diffent as well lighter ,able to carry greater loads without resistance heating
Seeing all this stuff may not give you an instant boost in technology you wont see SST's in 1950 but knowing it could be done is half the battle maybe ?
As for engines and avionics ? Far too complicated for me but I've watched avionics become so complicated in the last few years that it now has sub specialisations What Barnes Wallace would make of T.V screens in cockpits[ and in colour] I dont know
Any modern aircraft transported by ASB's into the past is FAR to valuable to be risked in war
I'm sure Im sure I've missed out hundreds of other things but that should be a taste