SHA-256 in FIPS 180-4 produces a 256-bit digest: 32 bytes or 64 hexadecimal characters. The 256 denotes output length, not Bitcoin block size or an encryption-key size.
The algorithm pads the message including its length, processes 512-bit chunks and performs 64 rounds on 32-bit words per chunk. These internal blocks are 64-byte computational pieces. A Bitcoin header is 80 bytes and a whole Bitcoin block also contains transactions: a different meaning of block. [NIST FIPS 180-4: SHA-256]
For a Bitcoin header, SHA-256 first hashes serialized header bytes, then SHA-256 hashes the 32 raw bytes of that result. It does not hash the 64-character hexadecimal representation. Output does not expand to 512 bits; constructions such as Taproot tagged hashes use SHA-256 differently. [Bitcoin Core v29.0: hash constructions]
For the three ASCII bytes abc without quotes or newline, SHA-256 starts ba7816bf8f01cfea and the full result has 64 hex characters. Adding a newline changes the message. Compare implementations using known test vectors and specify input encoding, hash count and byte-display convention. [Bitcoin Core v29.0: hash test vectors]
For the clearest picture, read this entry together with Cryptographic hash, Proof of Work, Block header, Bitcoin address, Merkle tree, Bitcoin. The reverse links also lead from Cryptographic hash, Hashcash.