I understand that https is enabled by default and i think i managed to set up the caddyfile in a way that this is enabled: https://caddyserver.com/docs/automatic-https
what i don't understand is how i configure the certificate to use. the documentation is very vague about that (https://caddyserver.com/docs/automatic-https), but i don't think caddy issues a valid let's encrypt certificate automatically.
You can use the tls issuer directive to specify how or from where the certificate should be obtained.
If you have obtained the certificate yourself, you can configure it like this in caddy:
tls /etc/ssl/mycert-chain.pem /etc/ssl/mykey.pem
You can also use the tls load directive to specify a folder with certificate and key bundle.
Here is the documentation on the tls directives: https://caddyserver.com/docs/caddyfile/directives/tls
This is not possible. Caddy only supports self signed certificates.