Just as a point of interest perhaps for folks who are not familiar with PCB design and modern electronics: the "huge" matrix of 21x21=441 LEDs would, with the specified LED from the bill of materials (BOM) cost all of $6.

That is based on the price for low quantity (1-500 pieces) though; if you were to build more than one board you would buy more LEDs, pushing the per-LED price (way) down. You can get 4,000 LEDs for $30.'

Edit: here is the LED in question, from the BOM [1].

[1]: https://jlcpcb.com/partdetail/C497920