Making the modelling is not very difficult. It is possible as well to use the equations to change the sizes of tubes. The image of the name IMG-20211018-WA0014.jpg shows you the sections at the corners of the Brace tube, not the surfaces. I don't understand why you want to modeling the welding. In my opinion, the sections of the welding will be irregular in shape. Exactly at the corners, the shape will be like in the photo attached, but you know nothing about contact surface to surface. The picture IMG-20211018-WA0014.jpg is showing you the point „a” what is the „Crown” point and the point „b” what is the „Saddle” point on the picture IMG-20211018-WA0015.jpg.
And to the point, it is possible of course to model the welding but why? SolidWorks has a special tool to show welding as a symbol.