What I'm describing is literally how this rocket was made... literally the whole thing is being live streamed by people living nearby with both 24/7 cams on the facility and frequent higher quality closer films and pictures being taken. Occasionally there are tweets from Elon about the process too.

You are far from the only one who is skeptical that this is a valuable process, but there isn't any room to debate that it is what they are doing.

Edit: Here is a recent timelapse of some of that welding, as an example: https://www.youtube.com/watch?v=J7bIvLhI1hY

I keep thinking of the story "Have Space Suit—Will Travel", except it now it is "Have Welders—Will Construct Space Craft". What SpaceX is doing is both crazy complex rocket science, yet it's also quaintly pedestrian. It makes space feel more accessible to me.

What's a rocket but a tractor with a few tens of thousands of lbs of fuel and oxidizer strapped to it?

There's a reason SpaceX is eating "big aerospace's" lunch and tests like this are part of it.