Self-signed certificate

What is 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.