I’m not sure I understand the question fully, but I’ll try to answer anyways.
First, what a 51% attack is, is when a single miner controls over half of the hashpower on the network. If an attacker who had 51% of the hashpower wanted to attack the network, they would simply continue to mine their own blocks, and refuse to build off of blocks of any other miner, and because they control over half of the hashpower, their version of the blockchain, consisting entirely of their own blocks, will be the longest. The remaining 49% of miners won’t be able to outrace them, and because Bitcoin considers the longest version of the blockchain to be the valid one, the blocks of the remaining 49% will become orphaned.
Because they can prevent any other miner’s blocks from being accepted, they will be able to include only transactions they want in their own blocks, censor transactions they don’t like, etc. since they control all new blocks.
As such, it’s not exactly that the Bitcoin network is reaching out to nodes to validate transactions. Other miners simply can’t catch up to the new blocks of the attacker, and so the attacker’s version of the blockchain is accepted as “truth”.