I heard that 100 passenger flights were operated over a few month period.

The NASA testbed had different engines. I think they were the same engines as the Backfire bomber and that the lease stipulated that NASA couldn't look at the engines. The tests were done in Russia.

I think it's unfair to say the Tu144 is a copy of the Concorde. Some common solutions were used, but Tupelov engineers had to design the retractable nose and other features, not BAC or Sud (now Aerospatiale). It may be like the Lexus being a copy of the Mercedes E class. Some similarities, but not a copy and no common parts used.

Was the F117A stealth fighter a copy of an alien design? Did the Roswell crash alien sketch a F117A during questioning?