What does “you got it” really mean here?

Can I use the phrase "annually" instead of "every year"?

Yes, I believe you can use "annually" instead of "every year" here. However, you would have to change the sentence slightly for the sentence to make sense. It would have to be "Thousands of Canadians lose a limb annually." Ex: We go on a camping trip every year. = We go on an annual camping trip. Ex: The convention takes place in Los Angeles every year. = The convention takes place annually in Los Angeles.

What is the nuance of "come on" in this case?

"Come on" here is used to express disbelief or show that you don't think someone is being serious about something. It is similar to saying "yeah, right" in a sarcastic way. It can also be used in this way to express annoyance. It is usually said in a slow, sarcastic tone. Ex: Come on, you can't be serious, you're just being dramatic. Ex: Oh come on, it's not that bad, just try it!

What is the difference between "cover one's eyes" and "close one's eyes"?

Closing one's eyes refers to the action of closing your eyelids, whereas to cover one's eyes means to use something (like a hand) to block out vision or sunlight. As shown in the video, the speaker has covered his eye with his hand to prevent himself from accidentally seeing something, which is a different action than simply closing his eyes. Ex: Close your eyes and go to sleep. Ex: He covered his eyes with his hands.

What does "got it" mean here?

Good question! You'll hear this expression in the sports context a lot. It's an exclamation used when a play has been successful. For example, when a ball has been caught. You might also hear "got him/her," which expresses that a player on the opposing team has been stopped or caught. In any case, it's used to express a successful play. Ex: Got it! He got the ball! Ex: Got him! Let's take the others down too.

It seems like the sentence still makes sense without the word "necessary." If I say "you're less motivated to do the actual hard work" instead of "you're less motivated to do the actual hard work necessary", what gets different?

You are correct. The sentence does make sense without the word "necessary." Both sentences have the same meaning. Having "necessary" makes the sentence a little more formal. Since this gentleman is giving a speech to a larger crowd, he may have wanted to sound more formal. Regardless, you would be fine using this sentence with or without "necessary."