Back in 2021 some guys in China claimed to have been able to upgrade the PoP RAM on a base M1 to 16GB: https://www.macrumors.com/2021/04/06/m1-mac-ram-and-ssd-upgr...

I remember when this came out, but more recently I recall finding something indicating that this was either untrue or pointless-- for example, while the ICs can be changed out, either the SoC or board knows what SKU it is due to an e-fuse or config locked into a cryptographically-signed firmware package, and refuses to address a different amount of RAM. I can't find that reference, however.