I know I’m a newbie, but I’ve worked on a few toilets along the way and this was a head scratcher for me.
There is what appears to be another supply line that runs up through the tank bolt and connects to the copper fill tube. The toilet operated ok, I just couldn’t tell what that second line was doing when I would flush.
Any feedback would be appreciated!
What the heck was someone thinking?
I guess both lines are cold water… perhaps it’s a warm H2O tank… 
That second line is screwed onto the bolt. Unless there is a hole in the bolt, the line is “capped” and doing nothing. Maybe it was a place holder.
That’s what I thought, but you can see the line coming up through the smaller piping on the overhead photo, which then connects to the fill line. (the other fill line I guess)
April Fools!!!
'Zactly what I was thinking! 
I find fools every month, not just April.