bitcoin back-end blockchain storage
Each non-pruning full node (peer) stores a complete copy of the blockchain - pruned nodes store a complete copy of the current state, but may not have all historical blocks available.

When a node validates a transaction or a block, it is only validating it against its local view of the blockchain - this means that looking up data is pretty fast, since all of it is available locally.

As the blockchain grows, access time for most data still remains mostly the same due to the nature of the key-value store used by bitcoind - look up times for a key value store are roughly constant, although data that is cached in memory can naturally be accessed faster.