And you are partially wrong.
An airframe
CONFIGURATION produces a unique radar signature. See the illustration above.
Airliners do not have wing sweep like jet fighters do -- that is one major contributor to a configuration.
Airliners have engines below the wings which produces a unique radar signature as shown in Fig 2 above. Jet fighters may or may not fly with external stores under their wings, therefore, an F-16's radar signature can radically differ from day to day, base upon what it carries -- or do not -- under the wings.
The B-52 bomber is comparable to the airliner in many major structures, which produces a similar radar signature to that of the airliner's radar signature.
You are partially wrong in that it helps immensely if the radar system have a library of known
COMMON CONFIGURATIONS of major airframes in the world. That way when it 'sees' a target
WITHOUT a cluster of voltage spikes like the examples above, it can immediately raise the priority level of that target. The target could be an F-15 or an F-16, but at least you can rule out an civilian airliner or a B-52 bomber. Do you understand?
You mean using the Doppler component of a radar return to track a 'stealth' fighter flying at 1900 km/h? That would be the 'Moving Target Indicator' (MTI) radar as an option.
You joined this forum May 2016. I posted a rebuttal to the MTI argument yrs ago...
https://defence.pk/pdf/threads/chinas-blitzkrieg-on-u-s-carrier.54955/page-29#post-3309403
That was in Aug 2012. If it was that easy to exploit the Doppler component (MTI) as a 'counter-stealth' method, that would have been proven after the F-117, and the F-22, F-35, and the B-2 would have never came to be. The MTI counter-stealth argument was nothing more than a fantasy cooked up by various anti-US Internet trolls ignorant of basic radar detection principles, and used more to attract attention to themselves than to enlighten the readers.
The MTI method does not work.
Then why are Russia and China still trying to deploy their 'stealth' fighters?
If it is that easy to defeat 'stealth', especially with the AESA technology, that mean
ANYONE can defeat 'stealth' on the cheap. So why are Russia and China struggling to deploy their 'stealth' fighters?
But I will say this about US -- that we have effectively defeated 'stealth'. Forty+ yrs of flying various publicly known and unknown 'stealth' aircrafts provided US with plenty of data on how a certain structural layout will produce a certain RCS signature. Your China is still trying.
The J-20 is Dead-On-Arrival (DOA).
Which was %99 useless to China.
The current low radar observable technique is shaping. If you do not have the
FINAL shape, then you are gambling, and when that F-117 crashed, its final shape with that low RCS value was essentially lost to China. The curvature method on the B-2, F-22, and F-35 are superior to the angled faceting method used on the F-117.