Is the minimal amount/percentage of
nodes that need to agree configurable in a blockchain genesis file?
As far as I can find there is no setting for this, and I assume this is set somewhere in the sourcecode?
That's not how consensus works in Bitcoin, as that would be highly vulnerable to a sybil attack.
First a full node validates all aspects of all blocks (incl. transactions) it receives. Anything invalid is discarded and not further considered.
Second the node picks the chain with the most total Proof of Work in it*. Usually there's only one, but sometimes two miners find a block at roughly the same time, so the blockchain temporarily forks until the next block increases the total PoW for one of the chains and all nodes will converge on that chain.
TL;DR The valid chain with the most total Proof of Work
*) Often referred to as the "longest chain" or better "longest valid chain", although strictly it's not about the length, but about the aggregate difficulty.