What seems to be happening is root shock or root damage. Overwatering will not help. Just keep it damp but not drowning. There were will be die backs as a result. The best think you can do is wait to let the plant heal. Remove any branches that die. The important part is the trunk portion.

Is the hole where this tree is planted properly made? Was the ground properly broken up? If the soil is clay and all that was done was to dig a hole, then what you have is bowl that will hold water. If this is the case, the roots are drowning and rotting away. Best to check if this is the case. Improve the drainage as soon as possible.

If the soil is sandy - did you amend it with compost so there's a bit of water retention. Don't let the plant go too dry between watering.

Also in such dry and withering heat (or extreme winter weather), it would be useful to add some mulching to keep the moisture/temperature level uniform. This only for its first year. Once establish you don't need to do this anymore.

Never fertilize in really hot weather, and never fertilize a newly transplanted plant. High nitrogen fertilizers can burn and destroy roots.

I bought the same yellow magnolia last year .
I live in Barrie further north than yourself

I had planed to plant it in the front yard to replace a tree that I want to be rid of.
But the tree in front, was not cut down and is yet not done.
I put the magnolia in my hospital or trial area. It has come through winter just fine and looking good
I have this small area that has great soil and I put in plants of different sorts. Sometimes something I want to try, or something I want to grow bigger before putting it in the perennial bed. Or something not doing well etc.

Tree is okay where it is for another year but am hoping in the fall to get that tree out in front removed.

Hope you can save your plant. They are just so gorgeous.