How is such an ad made?

How is such an ad made?
https://de.aliexpress.com/item/4000916541571.htm
Is this a multi-layer board?

You mean this thing:

You can buy a handful of them for a few EUR and rip them apart to see what’s inside.

Yes, I’m interested in such an ad. I need them for my solar light project. The lifepo4 3.2v 86Ah lithium battery has different values than a 3.7V 18650 battery.
Parameters: (note: N represents battery quantity)

  1. When battery voltage is over N*3.2V, it will illuminate 1 block current amount.
  2. When battery voltage is over N*3.4V, it will illuminate 2 blocks current amount.
  3. When battery voltage is over N*3.6V, it will illuminate 3 blocks current amount.
  4. When battery voltage is over N*3.9V, it will illuminate 4 blocks current amount.
  5. When battery voltage is less than N*3.2V, 4 blocks display will be off; it represents battery is less than 3.2V, and you can charge the battery.

Would a series resistor suffice for this?

I do not know how that thing works.

If I would make something myself, I’d put a microcontroller into it.

A simple old-school way is to use a voltage reference, voltage divider ladder and 4x comparators to drive the LED:s.
There are some dedicated analog IC:s too.
Modern way for those who like software is to take some small µC with an analog input and drive the 4 LED:s from that.

Here’s a model where the IC on the back can be seen:

In the others there may be a chip under a blob.

So I scrapped such an ad first.
When cutting apart, I cut off two components at the bottom. But now I know how the part is constructed.
The stupid thing is I can’t find a product description for the IC. This is from “STMicroelectronics” and has the designation “MZ15925”. I see the next problem in replacing the SMD resistor 0402 inc.

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