What should I enter here for my domain?

I use Sendgrid for one email and Sendinblue for other three emails for my domain, for sending emails (SMTP) only, what should I enter here?

So you use multiple outbound e-mail services to send e-mail from your domain?

For a specialized situation like that you should probably create the DMARC/SPF/DKIM records manually through the DNS panel rather than trying to use Cloudflare’s “wizards”

you should only have a single SPF record, but in can reference “include:” statements for multiple services. Check the documentation for your e-mail services, see what they recommend for an SPF record, then stitch them together (check the SPF specification if you’re uncertain)

DKIM I haven’t messed with much, check this Can I Have Multiple DKIM Records on My Domain? - DMARCLY

DMARC you should have only a single record, and it can largely be whatever you want it to be as long as it complies with the DMARC specification. Use a DMARC builder tool if you’re not sure. Example: DMARC Record Generator - Create DMARC DNS Records - MxToolbox

Will check into it, am not into coding at all. Learnt, designed and hosted a website for a pro-bono service, so kinda a noob in this domain.