https://travel.gc.ca/returning/customs/bringing-to-canada/personal-exemptions-mini-guide Does this mean that if I buy an iPhone X in the US and bring it back to Canada, I would have to pay Canadian taxes on top of the US sales tax since it costs more then $800? If the answer is yes, what taxes am I paying? Both US sales tax and Canada sales tax?
When you buy in the US, you pay the state sales tax on the item. The sales tax differs by state and some have no sales tax at all. Wikipedia has a summary which might be out-of-date but do note that some states have no sales tax. There you would pay no tax on your US purchase. Google lists 5 such states.
When you enter Canada with more than your exemption, which can be up to $800 CAD, you pay duty on the full amount, not just the part that exceeds $800 CAD. Duty is not the same as Canadian tax, which varies by province since it is made up of a Federal and Provincial component, although some charge it as a single percentage as a HST (Harmonized Sales Tax).
Duty depends on the type of item and its country of origin. There are certainly detailed documents that allow one to know exactly what the percentage is but as of last year, I paid roughly 15% of electronics bought from stores in the US. Many online US stores do not charge sales tax when shipping to Canada, so you can sometimes get an better deal by buying from a US store while in Canada, taking into account the cost of shipping, if applicable. A few stores now offer Customs Pre-Clearance which allows them to compute duty and show you the total before you complete the purchasing process which makes it easier.