When unconnected, the inserts act as an output right after the micpre that is connected back to the input of the channel. (depends on board!! some inserts are after EQ!) When connected, you take the output, route it to some gear (mostly compressor/gates) and feed it back to the input.
The inserts need special cables, the tip and ring function as input and output, through the same cable!! Now if make a special cable, that connects the tip and ring on one side, and has a mono jack on the other, you can just get the signal out of the board, without interrupting the channel...
And for your original question: you will also need mics, stands, amp, monitors, headphones etc for monitoring, alot of cables, preferably some effectprocessors (for reverb etc)... Don't underestimate the cost of these 'smaller' parts!