These are really great questions, taxes can be difficult to navigate even on the best of days.
The first thing I would recommend you to do is to speak to a local tax official or accountant in your region to see which taxes you need to pay or might be liable for. I did recently write a US sales tax FAQ which might provide some more information for the US sales tax side of things, but outside of that, I am not completely sure. There's a section for international stores outside the US and how they might be liable:
With regards to Canadian and UK sales tax liability for international merchants, I recommend doing some research and speaking to your local tax official. I can't stress that enough. Shopify also has a guide for setting up taxes for stores based out of Japan which can be seen below: