Mailchimp is a marketing automation platform and email marketing service that supports DMARC compliance through DKIM alignment for its customers. We've created a guide to provide you with all of the necessary steps and considerations for configuring Mailchimp: https://dmarcian.com/mailchimp-source-guide/
See our guide to configure DKIM.
Mailchimp often makes use of its own addresses in the mail-from in order to perform actions such as bounce management (failed delivered, rejected mail, etc.). For this reason, adding Mailchimp’s include statement to your SPF record is not necessary because it will not pass SPF alignment. Your domain should not have an SPF entry for this sender.