SubjectPublicKeyInfo is a SEQUENCE of an AlgorithmIdentifier and the subjectPublicKey BIT STRING. For EC keys the algorithm is id-ecPublicKey with the curve OID as parameter and the bit string holds the SEC 1 point; for RSA it is rsaEncryption with NULL parameters and a PKCS #1 RSAPublicKey inside; for Ed25519 it is 1.3.101.112 with no parameters and 32 bytes. DER sizes: 91 bytes for P-256, 294 for RSA-2048, 44 for Ed25519.
It is the interchange format for public keys: OpenSSL -pubout, Java's X509EncodedKeySpec, Go's x509.MarshalPKIXPublicKey, WebCrypto's spki export and most JWT libraries. Hashes of the SPKI are used as key-level identifiers that survive certificate renewal: HPKP-style pins (pin-sha256), Certificate Transparency monitoring and Chrome's CRLSets.