These are the two functions for the two files (from OpenSSL docs):
SSL_CTX_use_certificate_chain_file() loads a certificate chain from file into ctx. The certificates must be in PEM format and must be sorted starting with the subject's certificate (actual client or server certificate), followed by intermediate CA certificates if applicable, and ending at the highest level (root) CA. There is no corresponding function working on a single SSL object.
SSL_CTX_use_PrivateKey_file() adds the first private key found in file to ctx. The formatting type of the certificate must be specified from the known types SSL_FILETYPE_PEM, SSL_FILETYPE_ASN1. SSL_CTX_use_RSAPrivateKey_file() adds the first private RSA key found in file to ctx. SSL_use_PrivateKey_file() adds the first private key found in file to ssl; SSL_use_RSAPrivateKey_file() adds the first private RSA key found to ssl.