You're probably running a (very) old version of Certbot. Switching to the snap installation method instead of the now defunct wrapper script means you'll always be running an up to date Certbot, as snap updates Certbot automatically.
Running old/ancient versions of Certbot means you won't have bugfixes, security fixes, new features and/or the possibility something will become incompatible in the future (not likely, but possible).
Also note that the --classic option is just an option for snap to install Certbot. It won't be used in the actual certbot command.
The certonly subcommand is not necessary when using the renew subcommand. In fact, both ar a subcommand and you can only use one of them: they are mutually exclysive.
Frankly, I would very much recommend you to read the actual Certbot user guide at User Guide — Certbot 2.6.0 documentation. Because I have the feeling you have, no offence, absolutely no clue at all how Certbot actually works.