What is there to understand? The Tristate output never comes into play because pin 19 on both chips is hardwired to ground. That pin needs to see a high to put the outputs into the floating state, and it can never get there. The way things are set up on IC1, whatever goes in on an "B" input comes out on the corresponding "A" output. IC2 is set up via jumper P1 to be just the opposite, Data in on A lines, out on B lines.
The thing in the lower right is a "title block". It gives information on who made the drawing, when it was drawn, what it is used on, etc. That one is pretty basic. On mechanical drawings, they also include stuff like scale, surface finish, material specs, etc.