I don’t think the answers provided answer what you are asking.

The bitcoin software is run independently on each user’s computer. Some people will even use custom software to use bitcoin, although their software will need to follow all consensus rules to get an accurate representation of the blockchain.

When accessing the network, each node will connect to random nodes (this is the default behavior, although nodes can have custom settings to connect to only specific nodes, or specific nodes and additional random nodes), so if one person’s computer is compromised, anyone connecting to that node will immediately be able to know they are providing invalid information about blocks/transactions. Each person’s computer will locally validate each transaction inside of each block and the block itself.