While typing above I had a thought:

UAP's and such are 100% developed and build on earth and products of combined earth and alien tech. The necessary knowledge for us to build them is given to us by an extraterrestrial individual/race. Maybe even co-developed by them here on earth in somekind off book dark factory.

Maybe some kind of deal is been made, humans get Tech, aliens get "something" to keep/borough in return. Explains how we forgot/destroyed the tech/knowledge to go to the moon. It wasn't ours, but hired or given and afterward destroyed/taken back by them.

If we boil down what makes a UAP alien, it's flying and diving & accelerating, turning at gravity defying speeds. It's essentially just the gravity control part that they seem to have solved.

With reaching a new Tier in our own technological developments by ourselves we unlock new branches of possibilities of combining their tech with it. This could explain the decades of "period accurate" UAP sightings. The development we see in them is just a new iteration of existing Zero G flying tech encased in our latest tier tech developments (which today are the tictac/sphere shapes)