mico:iBOM

[kose2ik]

1. What is this "U3" Mfr. No (or. Mouser Part No.) ? "W25Q128JV" (Footprint:"PSON50P300X200X60-9N")
   (Density 128Mb, Frequency 133MHz ?)
2. [W25Q128JV_DTR] ( https://www.winbond.com/hq/product/code-storage-flash-memory/serial-nor-flash/?__locale=en&selected=128Mb#Density )
3. W25Q128JV-IM/JM(2) valid part numbers: W25Q128JV-IM, ,
   *S, SOIC-8, 208-mil, 128M-bit,
   PRODUCT NUMBER W25Q128JVSIM,
   TOP SIDE MARKING 25Q128JVSM ?
4. What is in photo, this chip top site marking ?
5. What is this "U3" (real) package type? (USON8 3x2 mm , USON-8-EP (2x3) , LGA8 3x2mm) (
   ( https://www.infineon.com/cms/en/product/packages/PG-WSON/ )
   PG-WSON-8-1 | PG-WSON-8-4 is 3x3mm,
   PG-WSON-10-800 | DFN-10 (002-10864) is 3x3mm,
   PG-WSON-14-800 | DFN-14 (001-96312) is 2.5x3.5mm)
6. Can I use too Mouser No:363-GD25Q128EQIGR , Mfr. No: GD25Q128EQIGR ?
   ( Package USON8 (4x4mm, 0.45mm thickness) 128 Mbit, 2v7-3v6 25mA 133MHz 16Mx8 8bit )

I'm N00B, first week learning KiCad 7.0 aso.
I want make this as thin as possible, long version ( RP2040 chip width and PCB 7mm ~ 8mm width )
(w battery aso. (maybe 2x mic (one audio & another case noise (after some software cleaning/canceling case noise out in audio) ))).

[aitesam961]

U3 is the 128Mbit flash that stores the program instructions as well as data. It is needed with RP2040 to work properly (uses QSPI) protocol.

Your idea of reducing PCB width is great but I'd suggest leave enough space on the sides of rp2040 so that you could route. There isn't a lot of space on the inner side of chip to route (barely power is routed).
If you plan to spend a bit more on advanced PCB fabrication then you can use vias next to each pin (on the sides ) to make things routable

[aitesam961]

Your layout drawings look understandable but please do not take the XTAL stuff too far away. Secondly, the two traces of XTAL should have the trace length close to.each other. Though XTALs running at such low freq. Doesn't require length matching but i experienced it is important to keep them at least closer. It can cause you trouble in bringing it up. Try to keep it closer and the traces should be clean and smooth without vias and interruption. Clocks are important :-)
Wish you best.