Skylights , the dreaded topic

[Roy Guard]

is it a requirement for the corner joints to be soldered and/or sealed?

[Jim Robinson]

Not that I am aware of. Certainly good practice to seal them but I don't think I have ever seen a field built flashing soldered. The sealant would tend to be under the metal so mostly would not be visible.

[Markus Keller]

That open corner should be clipped in under the other piece so that it keeps the corner closed. That's why it has that U hook type end.
Never seen anybody solder the corners. That flashing is mostly about water run off not so much sealing. The sealing is done by the stuff underneath.