It’s serverless though, right?

no, and in fact the cost of running it is really high because the server infrastructure they need to pay, they even say it themselves "Signal is expensive" https://signal.org/blog/signal-is-expensive/

it would be possible just to fork and use a “European” version of it

in theory yes, in practice no one has done it, and then you should not use Signal but the european fork which will not be compatible/federated with signal