If these crashed craft exist, and are damaged, then full reverse engineering is most likely not possible due to materials which do not exist on Earth as well as probably not being able to fully understand it with current science. However, some aspects might be able to be copied or inspire new technology or designs.
Also, if these craft crashed in the first place, then whoever built them could still be using others.
you know nothing about nothing. the tech you think exist and you might use, is 50-70- years old.
crystals grow, and in a geometricall perfect shapes,with right angles. why do you think that is? how can you cut a crystal and its perfectly clean on its edges? no shards. crystals grow.
your dumb ass phone has a liquid crystal screen.
emails, phone, all used in WW2
you know nothing. im not saying i know, cause that would be imbicile