put bike back together and now it won't turn on

That yellow wire appears to be your main power from the battery positive post.
It goes through a green fuse holder and then to the red wires and out to the main systems. It should be under the bolt on the battery PLUS terminal.
This is not factory stock so all the colors changes are confusing.

I think what is confusing is that he replaced the battery and the nut and bolt is still the pieces off the old battery terminal.
Someone has replaced the old style flimsy 30 amp fuse with this set up.
If the yellow wire is connected to the positive post the all circuits will have power.
The only thing on that post now is the heavy cable to the starter solenoid. It also needs the power to all the ignition and light circuits. My 1983 750 has the same configuration and I replaced the old fuse which had cracked with a new fuse holder. It would be nice to clean that up with a new 10 gauge RED wire and in line fuse.

If you touch that yellow wire to the positive terminal I am sure you will then have power to all your circuits.
Those red wires are the main feed wires and so they need to be powered by the battery positive post. The main 30 amp. fuse is there to protect against heavy overloads.
That is the same wiring on my 83 750 .

Good to hear you have it figured out. I am no expert myself and I am still learning. But what I have found in the manual I like to help other with.