Hey Wael,
He does give you a glimpse of how it is all done. If you continuously halt/pause the YouTube movie, you can see what buttons he is pushing to get where he is going. Making notes as he does this, pausing alot, will show you much of how he does this.
One odd thing he is doing in this tutorial is exporting his canopy and then the fuselage to a custom 'own' folder, then importing it back again with 'merge' (sort of like import procedure). I havent heard of that and it seemed redunent to me.
Instead, what he could do is 'Edit/Clone' the fuselage and name it Canopy. Then Edit/Clone again and name that one Fuselage, then rename the older fuselage 'original Fuselage.' Next, I would cut the 'canopy' from the 'canopy' fuselage, and the fuselage from the new fuselage part using Boolean.
BUT!!!!!!!!!!! In my years of doing these things, I found that Boolean creates a zillion little 'extra' Vertices... If I use a template 'line' and I cut the edge manually using Editable Poly mode; Edge/Create and Edge/Divide, and delete unwanted edges, and add in Vertices into the middle of Polygons (for curves) using Editable Mesh mode and 'Polygon/Divide' (to add Vertices), then back to Editable Poly mode; 'Edge/Delete' and Edge/Create to form the new lines (curves, etc), I then can create a new cut line for a nice canopy, precisely, without dealing with the unstability factors of Boolean, and 'no' (none whatsoever) extra Vertices...
To quote the old Alka Seltzer commercial; 'Try it, you'll like it....'