In a production environment, a real SSL certificate is used, but in a test environment, it is troublesome to obtain an SSL certificate just for HTTPS testing, so you may want to use a self-SSL certificate. I have.
The location of the self-SSL certificate on Ubuntu is: The private key for the self-SSL certificate and the self-SSL certificate.
Self SSL certificate /etc/ssl/certs/ssl-cert-snakeoil.pem Private key for self SSL certificate /etc/ssl/private/ssl-cert-snakeoil.key
Apache self-SSL certificate and self-SSL certificate private key setting
It is the setting of the private key of the self-SSL certificate and the self-SSL certificate in Apache. You do not need to specify an intermediate certificate file.
SSLEngine on SSLCertificateFile /etc/ssl/certs/ssl-cert-snakeoil.pem SSLCertificateKeyFile /etc/ssl/private/ssl-cert-snakeoil.key