I have a Minecraft Question

I have a Minecraft question, and as a total Minecraft noob, I think I do have the right to ask this...

After looking up some Minecraft vids on youtube, I keep seeing people with Cars, Airplanes and I even saw a video with guys having what seems to be guns!  Now I know that these are not normal in the MC game so I have come to the conclusion that they are Mods (Add ons) of some sort.  So my Questions are as follows

1. Where do I go to download these mods
2. Do they cost money
3. How do I put them on my game?  Like Make them active

Thats Pretty much it,  anyone who knows please comment!

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