Answer by Sergey Ponomarev for What is the file with .p8 extension? (APNs Auth Key / JWT)

This is a private key most likely in PEM format.The file type extension is described in IANA application/pkcs8.

The extension is defined in RFC 5958:

.p8 files are sometimes PEM-encoded. When .p8 files are PEM encoded
they use the .pem file extension. PEM encoding is either the Base64
encoding, from Section 4 of [RFC4648], of the DER-encoded
EncryptedPrivateKeyInfo sandwiched between:

-----BEGIN ENCRYPTED PRIVATE KEY----- -----END ENCRYPTED PRIVATEKEY-----

or the Base64 encoding, see Section 4 of [RFC4648], of the DER-
encoded PrivateKeyInfo sandwiched between:

-----BEGIN PRIVATE KEY----- -----END PRIVATE KEY-----

You can use Keystore Explorer to open it.