Yes, is uni-directional i.e. any given block has a pointer to the previous block (until you reach genesis). The hash of block N-1 is part of block N, hence it becomes part of N’s identity.
Is there one blockchain per node?
All nodes eventually find consensus about the chain state. There may be short lived chain forks (in any given node) due to latency in block distribution. With Cardano, every full node has a copy of the full chain state.
Is ledger a blockchain?
Ledger is a general term for a thing that you can only append to and cannot change historical content. Historically, book keepers used ledgers to record their (monetary) transactions. A blockchain is a distributed digital ledger.
What does Cardano do to prevent a blockchain running out of space?
If you mean disk space, currently nothing. The 308 epochs (i.e. 4.2 years) that we have seen on mainnet so far use up approx 15GB. A simple 1TB SSD card would last 286 years (at the current rate). Having said said that, permanent store space will become an issue for blockchain nodes and it is likely that these storage concerns will be handled by specialised storage nodes/solutions e.g. arweave
How does Cardano maintain efficient access, preserve performance, to blocks even as blockchain is growing?
Cardano performance is currently “network bound”, neither CPU nor storage I/O are an issue and likely never will be. The question is, how fast can you distribute a block of a given size around the globe so that nodes can find consensus. The term you might be looking for is “finality” i.e. how long does it take in realtime until a given Tx becomes permanent i.e. will be part of the immutable ledger history (and just just reside in an arbitrary fork that might get rolled back)
Binance currently wants 30 block confirmations for Cardano. With one block every 20sec, this would be finality after 10min. With Hydra scaling, this will likely/hopefully become a lot faster.