You cannot transfer a domain to Netlify, see:
https://answers.netlify.com/t/support-guide-how-do-i-transfer-my-domain-name-to-or-away-from-netlify/186
The good news though, is that you don’t need to.
Your domain can be with any registrar.
If you want to use Netlify DNS you would follow this documentation:
https://docs.netlify.com/domains-https/netlify-dns/delegate-to-netlify/
If you want to use the existing DNS you would follow this documentation:
https://docs.netlify.com/domains-https/custom-domains/configure-external-dns/