A .crt file is often the same as a .pem file, it's just called .crt so you know what's in the file. Same with .key. There are other encoding's like der (which you are trying to convert the pem encoded file to here), but you probably don't need that unless the service you are configuring explicitly requires it.
The PEM encoded files produced by certbot include:
- cert.pem - just your pem encoded cert, also the public key
- chain.pem - the other intermediate certs that make up the certificate chain (not including the root)
- fullchain.pem - your cert, plus the intermediates, this is often the file you need
- privkey.pem - your private key
You can convert or combine these files in different ways depending on how you want to use them, so the question is what type of service are you trying to use the cert for?