Self-signed certificate
# install openssl 4 ubuntu
apt install openssl
# install openssl 4 mac
brew install openssl
# self-signed certificate
openssl genrsa -out key.pem
openssl req -new -key key.pem -out csr.pem
openssl x509 -req -days 9999 -in csr.pem -signkey key.pem -out cert.pem
rm csr.pem
# https://www.sslchecker.com/certdecoder
For trusted certificate, take a look at Let's Encrypt and Certbot.