You bolt all the bits together. It goes at about 2,000 mph!
In reality it is a complex engineering problem made up of hundreds of engineering problems made up of thousands of difficult engineering problems.
It took teams of hundreds of people years to design and build the Blackbird and the same is true for any modern product. Very few things made today are made by one person. All things are built upon complex products.
Even if an individual built an app on their own there are still millions of people who built the mobile phone, software language, internet connectivity and power supply grid to support the app.
Sir Isaac Newton said he was “Standing on the shoulders of giants” and this is even more true today.
Iconic plane! look-up Kelly Johnson if you want to know more about how the SR71 was created – he was a very clever engineer. I would recommend his biography too, called “Kelly: More Than My Share of It All”. He was an engineer at the Lockheed Skunk works during the cold war and had a big input on several well known aircraft projects.