The cold water is entering in the left corner that we can not see in the picture, but the water can flow in the copper connection all the way up to the black arrow. From this connection there are 10 parallel copper pipes making the cold water (blue arrow) flow to the right. On the right side there is a similar pipe connection that extends all the way in to the bottom wall of the box. From this connection there is another set of ten pipes making the water flowing back to the second half of the first connection (yellow arrow). At the top of the picture you see the exit pipe (darker yellow) feeding the preheated cold water to the shower.
So there are a total of 20 pipes that looks like they have been pressed half flat and then soldered together to form one big copper area. The drain water from the shower will flow over this area.
The drain water from the shower enters the heat exchanger at the end of the big pipe on the right. I put a flash light in it so it can be more easily seen. The warm drain water will counter flow with the cold water coming in, which is the best way to do a heat exchange. The copper area is tilted 2 cm (about an inch) to make the drain water flow slowly over it all and exiting out the hole from where the picture has been taken. Since the cold water flows in parallel pipes, then there should be no problems with pressure drop in the cold water supply.
The next step was to connect it to the shower. Continue reading here: 4.Conecting the small heat exchanger