To build one, go to Campaigns > Create, choose Email, pick an audience or segment, build the content from a template or from scratch, and either send immediately or schedule it. Subject lines can be A/B tested across a small percentage of the list before Mailchimp automatically sends the winner to the rest, if you turn that feature on.
Resending to people who didn't open the first send is a built-in option from a completed campaign's report page, choose "Resend to non-openers," optionally tweak the subject line, and send again, usually a day or two later. Mailchimp caps how many times you can do this per original campaign, so check the current limit before planning a resend cadence.
Saving a campaign as a template keeps its structure and content reusable for future sends, from the campaign's options menu. Deleting, copying, and editing a sent campaign are all available from the Campaigns list too, with the caveat that a campaign already sent can't be un-sent, only resent or copied into a new one.
Mailing lists themselves (Mailchimp calls the single-list version of an "audience") are built from signups, imports, or integrations, and a segment is the dynamic filter layered on top (see our Mailchimp segmentation page → for the real mechanics there).