Usually by doing "Reply all" instead of replying to just one recipient. One of the addresses will be associated to the mailing list, and forward you message to everyone when you send it.

Basically, mailing lists are just a "special" email account everyone can send/receive emails to/from.

"Reply all" is the best single answer, but it also varies between lists -- on some lists, the norm is that you reply only to the list and don't CC the sender.

For example this is the norm on many Debian mailing lists. Or at least some people firmly consider it a norm, and will grouch at people CC'ing them.

The fact that this varies from list to list, and it's not encoded in software and you're just supposed to know which style to use, and that people then blame the human senders for not getting it right all the time, is among the reasons that mailing lists can be an unwelcoming medium.

The fact that you're getting downvoted is precisely the attitude that drives people away from mailing lists.

Traditionally, many mailing lists set the Reply-to header field to the mailing list address, so that replies would go back to the mailing list by default. For various reasons, that has fallen out of favor. Nowadays the message headers of mailing list messages usually indicate the list address in a separate header field, and mailing-list aware email clients provide a “reply to list” action. Unfortunately, too few email clients are mailing-list aware in that way. “Reply all” is the fallback, but make sure to remove the non-mailing list address, or else the original sender will receive two copies of your reply (one through the mailing list and one directly), which can be annoying.

I haven't been on a mailing list for a while. Do you know if gmail or protonmail do the "correct" thing?

Good question. I don't think there's a way to set mail headers on gmail or protonmail web interfaces. I'm under the impression some headers are required. Message IDs, I think.

On Thunderbird I get a button that says “reply list” that lets me do that. Failing that I guess you can just do a normal reply and then change the To: field to the list address.