How do I create notches for mating sheet metal to be joined by welding ?

Plug welding sheet metal then? Or perhaps a "Lego-design" so the parts are self mating/aligning?

Perhaps a Sketch to define the notches which correspond with said metal mating part(s) and a part level cut-extrude. (If you have a typical "notch" you could draw your first one and make it a block to then copy paste into additional sketches. This will keep all of the sketches within the blocks parametric)

Or in context edit of part needing the notches and offset the surfaces of mating part into part needing notches or trimming. This works well for curved surfaces and strange angles, etc.

However if you don't know how the welding process is performed it may be wise to learn a bit about welding and welding joint design before you attempt to place to much time on something that isn't practical per the industry. Especially with sheet metal design controlling distortion, warping and heat affected zones is critical for success.