What is going on with the PCB? It looks like the soldered several dev boards to a big PCB. I can't think of a good reason to do this.

A good reason to do it (IMO, as someone who has done it) that it's a lot faster, easier, and often cheaper than designing a fully custom board.

Not only would designing a fully custom board require a lot of error-prone effort, getting a lot of components in tiny quantities can be prohibitively expensive.

Good engineering includes optimizing for cost (monetary and non-monetary), and I'd argue that for many hobbyist one-off projects that won't be manufactured at scale, dev-boards-on-PCB is good engineering.

I've done the same thing for a few reasons.

- Certain things need good layouts - I think I can see a DC-DC Converter, you need to minimize certain loop areas, etc. You also need to ensure that you use the correct footprints for the chips. Way easier to just use a bunch of modules with a simple pcb.

- It reduces the components you need to aquire - you need to select a inductor with the correct specs for the dc-dc converter, a bunch of different resistors and capacitors for all sorts of uses or you just purchase a cheap module from china.

- It's easier to solder. Fancy chips with packages with no leads are not trivial to solder. LED's might be a bit sensitive to temperature and can get destroyed by a not-so-great solderer. Small pitches on IC make it easy to create solder bridges or bad connections.

Yes, you're nit the first telling me this. At first i wanted to make full custom pcb with smd components. But it cost me too much to make just one and some components were difficult to find, so i opted to make more like a shield so is more easy to find the componets and piu easier to solder than SMD for those who have never done it before. This is the same.The last one is also valid for the reason why it was written in Arduino and not in C, that at least even those who are not so familiar with it can modify it

Some good reasons off the top of my head:

It’s easier to do pcb-layout wise. Allows you to re-use most components between board revisions, or when you’re done with the project. Probably less ewaste overall because of that.

This is like asking a painter why the hell he used ready made paints and did not create his own for each color he needed. Not everybody has a hobby of spending time and money fixing tracing mistakes of PCB revision 11, playing with working contraption is fine as it is.

It's microservices, but for hardware.

I mean, yeah, why not? You take a bunch of off the shelf boards and put them together on the PCB as is. To make a PCB with all of the components on it would not be very DIY. That's a custom board and out of scope for DIY.