Hi Django57,
I learned leather-work from Al Stohlmans many books, I had no other way to learn it. Now I am glad I did, because I learned it the right way from the start. No bad habits or wrong techniques that had to be dealt with later.
My advise to you is to do the same. You do not have to buy all his books, buy the one called " Leather-craft Tools *How to use them* How to sharpen them* " by Al Stohlman, the grand ol man of leather-craft. It describes all the tools, how to use and maintain them. Then buy your self a good round or head knife and practice how to use it on scraps, you will not regret it.
I have all kinds of skiving knifes and tools, but if I had to limit my self to one knife; I would choose one of my small round knifes.
On internal corners I use the tip of the round knife. One pattern one knife, you do not want to stop and change knife; then you will have unwanted notch in your pattern. Do not forget to buy or make your self a stropping board, every leather knife has to be polished after sharpening and prior to use.
Clicking knifes (like the second knife in your video), is used to cut outside a cardboard pattern. I also use this technique on thin leather. On thick leather I draw around the pattern with a awl (round clicking awl) and let my head knife follow the awl trace afterward.
I bet more than 90 % of our members use the head knife and love it.
Thats my two cents.
Tor