United Launch Alliance has published a video of the fuel and oxygen tanks joining together for their next generation rocket called Vulcan.
Vulcan used liquid methane and liquid oxygen to fuel the two Blue Origin BE-4 rocket engines on the booster stage. To join the two tanks, ULA friction stir welded them together. This is the process of putting pressure on either side of the metal pieces to be joined together then spinning together to form one piece of solid metal.
This isn’t like regular welding where metal is melted to form a single piece which can cause a weaker, chemically different joint. This process forms a must stronger and chemically unchanged weld. ULA plans to launch the first Vulcan in mid-2021 with a payload to land on the Moon.
See it in action here:
And here more about this process below: