G Suite can be a bit confusing sometimes, but fear not, here is a quick guide to set your group up as an email list. I'm sure there is an official guide somewhere, but I was unable to find a definitive, official source for this information. First create your group, I usually select "Restricted" as the access level to ensure the group is not a public forum.
Next set your group access as follows to allow for public posting:
Next just to double check, go to the group moderation settings (link at the bottom of the g-suite group settings page, note you will have to add yourself as an owner to the group to see these settings):
Lastly, unless you want any groups to be publically accessible, go to the G-Suite Groups for business app advanced settings and ensure your defaults are all set to private (unless emailed to group from outside), only members can see topics (unless you want all members to be able to see the group's emails), and I like to set it up so only domain admins can create groups. Use your discression on this part.
This process has changed in the past an it may change again, so there is no guarantee this guide won't be obsolete in another month, but hopefully it will help someone.