1st and 2nd option do the same.
No, they don't. It's only that since your video does not any actual transparency there is nothing to retain. Read the help what each of the layer switches does.
To see a difference, you'd need a layer in the comp with an alpha channel.
Tell you what: instead of stumbling around in the dark, get a good foundation in the basics -- from the pros -- and life in AE gets a lot less frustrating:
Put a shape layer below the MP4 then toggle preserve transparencry and you will see the MP4 trimmed to match the transparency of the layers below. That's what it means to preserve underlying transparency.
Why still giving me links to the manual? I don't get it from manual either. So I am asking here.
I see also 2 tutorials on YouTube but I don't understand what is going on.
Here is even good one: https://www.youtube.com/watch?v=02jWF7A4p5E
But for me that option "T" is almost the same what track matte. Almost no difference. Maybe less or two or three mouse clicks.
But I never figure that out what Daves said. He said "you need a layer with an alpha channel" so I thought that I need a video with transparency background from the beginning (imported finished clip with transparency, for example uncompressed avi). So for my "with alpha channel" means just like I said above. I can't figure that out if you are so imprecise in your post.