SPF
Add include:spf.sendinblue.com to your existing SPF record. Do not create a second record: two SPF records mean neither works, and that is the most common mistake in this whole area.
DKIM
Found in the Brevo account, under Senders, Domains and Dedicated IPs.
Brevo publishes DKIM and Brevo-code records for verification. Both need to be in place before authenticated sending works.
Brevo publishes keys under a Brevo-provided selector, which is what an external check looks for.
DMARC
DMARC is the same regardless of provider, because it is your instruction rather than theirs. Start at v=DMARC1; p=none; rua=mailto:you@yourdomain.com, read the reports, authorise everything legitimate, and only then tighten. The DMARC guide covers the order, and the order is what stops you blocking your own invoices.
Worth knowing with Brevo
Brevo is commonly used for marketing mail alongside a separate provider for ordinary correspondence. Two senders means two SPF includes, and that is how lookup counts creep upward.
Watch the lookup count
Every include costs one of your ten SPF lookups, and so does every include inside it. Adding a third or fourth sender is where domains typically cross the limit, after which the whole record fails silently. See the lookup guide before adding another.