TechNestSPF asks who sent it. DKIM asks about the letter itself. The server signs the letter. The other side checks the signature with the public key on the domain. With no key, the signature hangs in the air.
The mail service gives you the key when the mailbox opens. You do not invent it. Hosting mail and a separate business mailbox use different keys. You add both as TXT records. You do not write one in place of the other. The detail is on the mail card.