To weld a patch section(around the wheel well) you need to butt weld the two pieces together. This is the correct way and it takes more prep work to get it to fit correctly before welding. Body shops usually opt for the quick and easy rather then spending the time to butt weld it. Also takes more of an experienced person to do it. You can use magnets to hold the panels and tack them or use these...
\ These can be hard to use on curved surfaces where a magnet will work better. When done correctly you should not even know the panel was welded. If you are installing a complete panel and don't have a spot welder plug welds are the best bet. So you will need a hole in either the new panel or what it is getting welded to. If that's case you can just use an ordinary drill bit to drill out the spot welds and use that for plug welding. As mentioned you need a weld threw primer not just plain metal.
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