Making sense of the copy descriptor operator

Take the example of a user running the command ls >moo 2>&1 or ls 2>&1 1>moo while he is on his terminal (term). (Note that the 1 is implicit. 1>moo is identical to >moo.)

Firstly, understand that x>&y means: "Make output to x go wherever output to y is going right now."

Knowing that, let's draw a table of what happens at each step of a redirection. Redirections are read from left to right.

Make output to 2 go wherever output to 1 is going, which is the terminal. Then, make output to 1 go to moo.

fd |  ls  | 2>&1  | 1>moo |
   |------|-------|-------|
1  | term | term  | >moo  |
   |    \         |       |
   |     \        |       |
   |      \       |       |
2  | term | >term | term  |

Make output to 1 go to moo. Then make output to 2 go wherever output to 1 is going, which is moo.

fd |  ls  | 1>moo | 2>&1 |
   |------|-------|------|
1  | term | >moo  |  moo |
   |      |     \        |
   |      |      \       |
   |      |       \      |
2  | term | term  | >moo |

You should also read the more in-depth tutorial about redirection in general: Illustrated Redirection Tutorial.