Honestly, I'm not sure if the French really can't understand us or if it's just bad faith sometimes - I've been told by several people that when they tried talking to a French person in French, they were replied to in English, which is extremely insulting (I mean, why??). However, I've met some French men and women who had no problem understanding us.

But I'll clarify : there are only minor differences between written/formally spoken Canadian French and French from France - the rift lies in the informal register (again, slang and swear words).

If the topic interests you, this article explains the differences between Quebec French and French from France much better than I can :
https://www.talkinfrench.com/canadian-french-difference/

Yes! They're as different as, let's say, for example, English spoken in London and English spoken deep into the Scottish Highlands - maybe more different.

They just don't sound the same to the ear - a lot of words are pronounced differently, swear words and slang are VASTLY different, etc. French Canadian just evolved a little bit differently from Old French overall.

To give you an idea, when I went to France, people just wouldn't understand me unless I made an effort to sound like them. (But we Québécois understand the French just fine.)